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Chapter: XML and Web Services : Building XML-Based Applications : Interactive Graphical Visualizations with SVG

Powering Web Pages with SVG

This section discusses how to embed SVG graphics in Web pages and then how to make these graphics interactive in order to deliver interactive graphical visualizations over the Web.

Powering Web Pages with SVG

This section discusses how to embed SVG graphics in Web pages and then how to make these graphics interactive in order to deliver interactive graphical visualizations over the Web.

Embedding SVG Graphics in Web Pages


Listing 12.7 shows an XHTML Web page with an embedded SVG diagram. To view SVG diagrams, including the one in this example, in your Web browser you must first install a plug-in capable of rendering SVG—for example, the SVG Viewer from Adobe at www.adobe.com/svg/viewer/install.


LISTING 12.7  3MonthLoan.htm—Sample Loan XHTML






<title>Loan Visualization</title> </head>




<embed  src=”3MonthLoan_WithCss.svg”  width=”485”  height=”290”

type=”image/svg-xml”  border=”1”

pluginspage=”http://www.adobe.com/svg/viewer/install/main.html”  />






The key element in this Web page is the embed element, shown here:


<embed  src=”3MonthLoan_WithCss.svg”  width=”485”  height=”290”

type=”image/svg-xml”  border=”1”

pluginspage=”http://www.adobe.com/svg/viewer/install/main.html”  />


The value of the src attribute of the embed element provides the URL for the SVG docu-ment to embed in the Web page. In this case, the SVG document is in the same directory on the Web content tree as the Web page in which it is embedded. The width and height attributes, on the other hand, define the dimensions of the SVG embedded diagram. The type attribute gives the MIME content type of the embedded SVG diagram (in this case, image/svg-xml). The SVG diagram has a border of 1 pixel, as defined by the border element. In the event that the Web browser viewing the Web page does not have the capability to view documents of type image/svg-xml, the pluginspage attribute defines the Web page on the Internet to which the Web browser user may go to get an appropri-ate plug-in to view the embedded SVG document. In this case, the user is directed to the Adobe Web site (www.adobe.com) to get the Adobe SVG Viewer Web browser plug-in.


Adding Interactive Behavior


Another key advantage of SVG over other vector graphics formats is the ability to add interactive behavior to SVG diagrams. This section looks at a small cross section of types of behavior that may be added to an SVG diagram to make it interactive, including those listed here:












For each type of behavior, code snippets are given in this discussion. These snippets are additions to the various documents already presented in this section in order to give them the desired interactive behavior. The complete resulting documents created by adding all the interactive behavior discussed here may be downloaded from the Web site resource center for this book. Refer to these files to see how the code snippets in this section fit into the overall loan application. You may also load the file named 120MonthLoan_Advanced.htm in your Web browser to try out the interactive behavior. The complete documents with all the interactive behavior discussed in this section include the following:


   120MonthLoan.xml. An XML loan document for a 120-month loan


   Loan_Advanced.xsl. An XSL style sheet used to transform an XML loan docu-


ment to get an SVG visualization


   120MonthLoan_Advanced.svg. An SVG visualization of a 120-month loan


   120MonthLoan_Advanced.htm. An XHTML document with embedded SVG visual-


ization of a 120-month loan suitable for delivery over the Web


120MonthLoan_Advanced.xsl. An XSL-FO document with an embedded SVG visualization of a 120-month loan suitable for printing




Scaling changes the zoom factor of the diagram. This enables the user to either zoom in for a magnified look at a subset of the diagram or to zoom out for an overview of the entire diagram. It is useful to enable the user to progressively change the zoom factor by a fixed delta. To implement this, we first add a form with the following input fields to the XHTML document in Listing 12.7:


<input type=”button” value=” - “ onclick=”scale( scaleFactor - scaleFactorDelta )” />


<input type=”button” value=” + “ onclick=”scale( scaleFactor + scaleFactorDelta )” />


It is also useful to enable the user to change the zoom factor directly to an explicitly specified value, as shown here:


<input name=”scaleFactorTextField” type=”text” value=”1.0” size=”3” /> <input type=”button” value=”Ok” onclick=”scale(

this.form.scaleFactorTextField.value  )”  />


These input fields catch the onclick event and make a JavaScript callback to the scale function in response. The value passed to the scale function is the new zoom factor, which is computed from two JavaScript variables also added to the same document and shown here:


var  scaleFactor  =  1.0;


var  scaleFactorDelta  =  0.1;


The scaleFactor variable holds the value of the current scale factor, whereas the scaleFactorDelta variable is the progressive change in the scale factor when the user is zooming in or out.


The scale function implements the interactive scaling behavior, as shown here:


function scale( newScaleFactor ) { scaleFactor = newScaleFactor; scaleFactorTextField.value = newScaleFactor; updateSvgLoanPlot();




This function updates the current scale factor and reflects that value in the text field in the XHTML GUI. It then updates the SVG diagram with a call to the following function:


function  updateSvgLoanPlot()  {


svgLoanPlot.setAttribute( “transform”, “scale(“ + scaleFactor + “) translate(“ + plotX + “,” + plotY + “)” );



This function sets an attribute named transform on an element in the SVG DOM to a value that contains a scale transformation with the new scale factor. This element corre-sponds to the single child “group” element, named g, in the SVG DOM, as shown here:


<svg height=”290” width=”485” xmlns:xlink=”http://www.w3.org/1999/xlink”> <g transform=”scale(1.0) translate(0,0)” id=”LoanPlot”>








The g element groups all the vector graphics elements of the SVG loan plot. Therefore, applying a scale transformation to this element causes the scale factor to be applied to every element in the SVG diagram. The JavaScript variable svgLoanPlot, which corre-sponds to the SVG g element discussed previously, is initialized when the Web page is loaded, as shown here:


svgDoc = document.embeds[ 0 ].getSVGDocument(); svgLoanPlot = svgDoc.getElementById( “LoanPlot” );


Note that the ID of the g element is LoanPlot. Also, note how this is used to locate the element in the SVG DOM. This technique may be used to locate other elements in the DOM with different IDs as well. Figure 12.5 shows a screenshot of the SVG diagram with a scale factor of 0.25 applied, causing a reduction in the size of the diagram to one quarter its default size. It is also possible to scale subsections of the SVG diagram through other vector graphic element groupings. Note that the default scale factor is 1.0, corresponding to no scaling. This type of interactive behavior is particularly important in SVG applications in the field of mapping. In these types of applications, the amount of detail visible in the diagram is changed according to the level of magnification of the diagram. For example, at the highest zoom factor, street names may not be shown because they would be too small to be legible anyway and would obscure other higher-level detail. For details of how to toggle the visibility of elements, see the upcoming sub-section titled “Highlighting.” Note that other transformations are possible, including rotations and translations, as discussed in the following subsection. For a complete list of possible transformations, see the SVG specification at www.w3.org.




Panning involves the movement of all or part of the SVG diagram up, down, left, or right (or some combination thereof). This type of interactive behavior is particularly useful in combination with scaling, discussed previously. For example, on an SVG diagram where the user has zoomed in for a detailed look at part of the diagram, panning may be used to “move around” in order to look in detail at different parts of the SVG diagram. The fol-lowing button controls may be added to the XHTML shown in Listing 12.7 in order to enable the user to perform the panning:

<input type=”button” value=” Up “ onclick=”pan( plotX, plotY - panDelta )” /> <input type=”button” value=” Left “ onclick=”pan( plotX - panDelta, plotY )” /> <input type=”button” value=” Zero “ onclick=”pan( 0, 0 )” />


<input type=”button” value=”Right” onclick=”pan( plotX + panDelta, plotY )” /> <input type=”button” value=”Down” onclick=”pan( plotX, plotY + panDelta )” />


When the user clicks one of these buttons, the onclick event is caught and new x and y coordinates for the translation are computed from the three JavaScript variables shown here:


var plotX = 0; var plotY = 0;


var  panDelta  =  25;


In this case, plotX and plotY values of 0 indicate no translation, and the SVG diagram is in its original position, where it first renders. The variable panDelta is the change in pix-els applied to either the plotX or the plotY coordinate for each step that the user pans.


Once the new values have been computed, a call is made to the JavaScript pan function, shown here, to update the SVG diagram:


function pan( newPlotX, newPlotY ) { plotX = newPlotX;


plotY = newPlotY; updateSvgLoanPlot();




Once the plotX and plotY coordinates have been updated, a call is made to the updateSvgLoanPlot function, shown previously in the discussion on scaling behavior. This changes the transformation applied to the SVG diagram to include the desired pan-ning. Figure 12.6 shows the SVG loan visualization with a scale factor of 1.75 applied, together with some panning, to show the first few data points of the plot.



In this discussion, highlighting behavior means toggling the visibility of SVG vector graphics elements. In this example, we show how the circles representing the data points in the plot may be toggled on or off so that they are either visible or invisible. A check-box may be added to the XHTML in Listing 12.7, as shown here, to enable users to tog-gle data points on or off:


<input  type=”checkbox”  name=”showDataPoints”  value=”on”  checked=”true”

onclick=”setDataPointVisibility(  this.form.showDataPoints.checked  )”  />


When the user either checks or unchecks this box, the onclick event is caught and the JavaScript function setDataPointVisibility is invoked:


function setDataPointVisibility( showDataPoints ) { var visibility = “hidden”;


if( showDataPoints ) { visibility = “visible”;




for( var i = 0; i <= loanTermInMonths; ++i ) { svgPaymentDataPoint = svgDoc.getElementById( “Month” + i ); svgPaymentDataPoint.setAttribute( “visibility”, visibility );





Just as the g (group) element was used in the previous discussions on scaling and pan-ning, it can also be used for highlighting. Each of the data points represented by circle elements in the SVG may be contained by a g element, as shown here:


<g  onmousedown=”showPayment(0,  0.00,  0.00,  263.62,  10000.00  )”>


<circle class=”datapoint” visibility=”visible” r=”4” id=”Month0” cx=”68.5” cy=”11.7”/>




The setDataPointVisibility function, shown previously, first determines whether the data points are being turned on or off and stores the result in the visibility variable. It then loops over each data point over the duration of the loan stored in the loanTermInMonths variable, which is initialized when the Web page is loaded (in this case, to the value 120). For each data point X, it looks up the g element with the ID MonthX, as shown in the SVG for Month0. The function then sets the attribute named visibility on this g element to either visible or hidden, depending on whether the data points are being turned on or off, respectively. Figure 12.7 shows the SVG loan dia-gram with all the data points turned off. This is another good example of how the SVG DOM may be manipulated by JavaScript functions to provide interactive behavior. This type of interactive behavior is particularly useful in SVG diagrams that include the con-cept of layering (for example, maps where the user may want to either view or hide a certain set of features in a layer, depending on how she is using the SVG diagram) .




This type of interactive behavior involves the user selecting an element of the SVG dia-gram to view more detailed information. To illustrate this concept with our loan example, we will enable the user to select any loan payment data point on the plot to view details for that payment, including the index of the month, the principal and interest components of the payment, the total payment, and the balance of the outstanding principal at the point of the payment. In this case, the event of the user clicking a component of the SVG diagram is caught by the payment data point of the SVG diagram, as shown here:


<g  onmousedown=”showPayment(1,  13.62,  250.00,  263.62,  9986.38  )”>


<circle class=”datapoint” visibility=”visible” r=”4” id=”Month1” cx=”71.7” cy=”12.0”/>




The g element enclosing the circle element used to represent the data point has an attribute, onmousedown, that traps a user click on the data point and calls the JavaScript showPayment function, shown here, in response:


function showPayment( month, principalPayment, interestPayment, totalPayment, principalOutstanding ) {


alert(  “Month:  “  +  month  +  “\n”  +  “Principal  Payment:  $”  +  principalPayment


+ “\n” + “Interest Payment: $” + interestPayment + “\n” + “Total Payment: $” + totalPayment + “\n”

+  “Principal  Outstanding:  $”  +  principalOutstanding  );




This function is in the XHTML document shown in Listing 12.7, in which the SVG dia-gram is embedded. The arguments to this function are the values of the detailed informa-tion for the data point. This function simply creates a JavaScript alert to show the detailed information for the data point in a pop-up window, as shown in Figure 12.8.


This type of interactive behavior is very important in a broad range of visualizations for which there may be a vast amount of associated information, but displaying it all at once would lead to too much information for the user to assimilate. Enabling the user to inter-actively request the specific information he is interested in helps the SVG visualization stay clean and effective.




In many interactive applications, the user is presented with information that she wishes to analyze (for example, by testing a few “what-if” scenarios and immediately seeing the effect). In our loan example, we illustrate this by enabling the user to specify an alterna-tive interest rate for her loan and have the visualization plot a curve showing the perfor-mance of the loan with the new interest rate, together with the original plot for comparison. The first step in this interaction is to acquire the new interest rate from the user—for example, by adding the form input fields shown here to the XHTML docu-ment shown in Listing 12.7:


<input name=”comparisonInterestRateTextField” type=”text” value=”10” size=”2” />%


<input  type=”button”  value=”Plot”


onclick=”plot( comparisonInterestRateTextField.value )” /> <input type=”button” value=”Clear” onclick=”clearPlot()” />


The first input field, named comparisonInterestRateTextField, is the text field in which the user types the new interest rate she wishes to try. The next input field is a but-ton labeled Plot that the user clicks to initiate the plotting of the loan performance for the new interest rate. Note that when the user selects this button, the form is not being submitted to a server. Rather, the onclick event for the button is caught and the JavaScript function named plot, which is embedded in the XHTML document contain-ing the SVG diagram, is invoked with the value of the new test interest rate. The last input in the form is another button, labeled Clear, that enables the user to clear a “what-if” test case plot. Similarly, this button causes the JavaScript clearPlot function to be invoked. The plot function is shown here:


function  plot(  interestRate  )  {


var  monthlyInterestRate  =  (  interestRate  /  100.0  )  /  12.0;


var monthlyPayment = round( principal * ( monthlyInterestRate / ( 1.0 - Math.pow( 1.0 + monthlyInterestRate, -loanTermInMonths ) ) ) );

var outstandingPrincipal = principal; var dataPoints = “”;


for( var i = 0; i <= loanTermInMonths; ++i ) { var interestPayment = 0.00;


var principalPayment = 0.00; if( i > 0 ) {


interestPayment = round( outstandingPrincipal * monthlyInterestRate ); principalPayment = round( monthlyPayment - interestPayment );




outstandingPrincipal  =  outstandingPrincipal  -  principalPayment;


testDataPointGroups[ i ].setAttribute( “onmousedown”,


“alert(  ‘“  +


“Month:  “  +  i  +  “\\n”  +


“Principal Payment: $” + principalPayment + “\\n” + “Interest Payment: $” + interestPayment + “\\n” + “Total Payment: $” + monthlyPayment + “\\n” +


“Principal Outstanding: $” + round( outstandingPrincipal ) + “‘ )” );



var cx = plotTopLeftX + ( i * ( plotWidth / loanTermInMonths ) ); testDataPoints[ i ].setAttribute( “cx”, cx );


var cy = plotTopLeftY + ( plotHeight - ( plotHeight / principal ) * outstandingPrincipal );


testDataPoints[  i  ].setAttribute(  “cy”,  cy  );


testDataPoints[  i  ].setAttribute(  “visibility”,  “visible”  );


dataPoints  +=  “  “  +  cx  +  “,”  +  cy;




testDataLine.setAttribute( “points”, dataPoints ); testDataLine.setAttribute( “visibility”, “visible” );




The first step in this function is to compute the monthly interest rate and loan payment from the given annual interest rate using the formulas presented earlier in this chapter. For each month in the term of the loan, this function computes the principal, interest, and total monthly payments, rounded to the nearest cent, using the simple round function shown here:


function  round(  x  )  {


return  Math.round(  x  *  100.0  )  /  100.0;




The detailed information for each loan payment is then set in the SVG DOM in order to show the loan performance for the new interest rate. In this case, the elements of the DOM that are affected are stored in the testDataPointGroups array, which is initialized when the Web page is first loaded, as shown here:

testDataLine = svgDoc.createElement( “polyline” ); testDataLine.setAttribute( “class”, “analysisData” ); testDataLine.setAttribute( “visibility”, “hidden” ); svgLoanPlot.appendChild( testDataLine );


for(  var  i  =  0;  i  <=  loanTermInMonths;  ++i  )  {


var testDataPoint = svgDoc.createElement( “circle” ); testDataPoint.setAttribute( “id”, “TestMonth” + i ); testDataPoint.setAttribute( “visibility”, “hidden” ); testDataPoint.setAttribute( “r”, “4” ); testDataPoint.setAttribute( “cx”, “10” ); testDataPoint.setAttribute( “cy”, “10” ); testDataPoint.setAttribute( “class”, “analysisDatapoint” );


var testDataPointGroup = svgDoc.createElement( “g” ); testDataPointGroup.appendChild( testDataPoint );


svgLoanPlot.appendChild(  testDataPointGroup  );


testDataPoints[ i ] = testDataPoint; testDataPointGroups[ i ] = testDataPointGroup;




This initialization creates a new polyline element as well as a g element that encloses a circle element for each loan payment data point. The attributes that are set on these ele-ments are similar to those for the current loan interest rate already discussed, with the important exception of the visibility attribute, which is set to a value of hidden to ensure that the elements of the test plot are not visible until the user tries a “what-if” sce-nario by specifying a new interest rate. Lastly, the new polyline and g elements are appended as children of the svg root element node in the DOM that is represented by the svgLoanPlot variable.

Given the initialization of our testDataLine and testDataPointGroups variables, as discussed earlier, the plot function listed previously modifies the attributes of these ele-ments in order to show loan performance for any specified interest rate. In particular, the cx and cy attributes of the circle elements, representing the coordinates of the center of the circles, are modified to move the data points into the correct positions in the plot to reflect the new interest rate. The visibility attribute of each circle is also set to a value of visible to make its data point visible. Recall that the plot is composed not only of data points but also a multisegment line represented as a polyline element in the SVG DOM. Similarly, this element is created during initialization and is set during a call to the plot function to reflect loan performance for a new interest rate. In this case, it is the points and visibility attributes of the polyline element represented by the testDataLine variable that are set to show the new multisegment line in the correct loca-tion on the plot and make it visible, respectively.


To clear a test plot for a new interest rate, the user clicks the form button labeled Clear (presented previously). This results in a callback to the clearPlot JavaScript function listed here:


function  clearPlot()  {


testDataLine.setAttribute( “visibility”, “hidden” ); for( var i = 0; i <= loanTermInMonths; ++i ) {


testDataPoints[  i  ].setAttribute(  “visibility”,  “hidden”  );






This function simply sets the values of the visibility attributes of each of the test data point circles and the underlying multisegment lines to a value of hidden in order to make them invisible. Figure 12.9 shows an SVG loan visualization with a plot of our default 30-percent interest rate, together with a plot for a test interest rate of 10 percent. In this case, the user has clicked the test plot to get a pop-up window with detailed information on one of the data points, as discussed previously.

This important example shows that interactive SVG visualizations may be realized not only by modifying attributes on existing elements in the SVG DOM, as shown previ-ously, but also by adding new elements to the SVG DOM, as shown here. In fact, it is also possible to delete elements of the SVG DOM, although doing this would have the same visible effect as making the element invisible, as discussed previously. This exam-ple also shows how the user may change the SVG visualization interactively in an arbi-trary way in response to input to realize any kind of interactive behavior. This flexible yet simple-to-use capability paves the way for powerful, new interactive visualizations using SVG. The next few subsections outline some of the key advantages of using SVG to add visualizations to content.


The Benefits of Web Pages Powered by SVG


SVG enables powerful and compelling visualizations to be created. This enables Web pages to be more interactive and engaging for the user. Although in many cases it is pos-sible to understand concepts through static information only, for many situations we learn more rapidly by manipulating items and viewing responses to our actions. Such sit-uations indicate good potential target applications for interactive SVG visualizations.


Faster Client Response Time

Many Web applications today are made pseudo-interactive through the use of client/server interaction. In this case, when the user performs some action on a Web page, the Web browser sends a request to the Web server to get another Web page that  reflects the changes associated with the user action. In practice, this approach results in unpredictable and frustrating application behavior, especially when the client/server interaction is occurring over a network without any guarantee of quality of service—a prime example of which is the Internet. In such cases, any user action is followed by a frustrating wait while the updated Web page is loaded. In the case of SVG, however, it is possible to realize true interactive behavior completely on the client side without any net-work requests to the server after the initial loading of the Web page containing the visu-alization. In practice, this leads to significantly faster client response time and more user-friendly interactive content.


Reduced Server Load


Because content that is interactive completely on the client side (without server interac-tion beyond initial loading) does not require subsequent server requests to deliver inter-active behavior, there is less demand on the server. This enables both improved performance for existing clients and improved ability to scale to service more clients.


Improved User Privacy


Many useful interactive applications involve the user entering and manipulating sensitive information (for example, financial information associated with a loan).


Where applications are pseudo-interactive, as defined earlier, this information needs to be exchanged with the server. Where the network connection between the client Web browser and target Web server is secured with HTTPS, some assurance is given to users that their sensitive information will not be compromised while in transit over the public, untrusted Internet. However, in many of these applications, this assurance may not be enough to engage users because they may not even want their sensitive information to propagate to the target Web server, where they fear it will be compromised or used against them (for marketing purposes, for example).


SVG, however, enables completely client-side interactive behavior that allows users to enter and interact with their sensitive information totally on the client side with no need to share this information with the server. This enables users to gain a better understand-ing of their options and make decisions before opting to share their sensitive information, such as in the case of a loan application.

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