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<A HREF="toc.html">Contents</A>
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<HR NOSHADE>
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<H2><A NAME="8_17">8.17 Using callbacks for Plot marks</A></H2>
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<P> An interesting enhancement when using Plotmarks is the possibility
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to add a callback function to control the size and color of the
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plotmarks.</P>
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<P> This callback function will get called with the current Y-value (for
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the plotmark) as it's argument. As return value the callback function
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must return an array containing three (possible null) values. The
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values returned must be</P>
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<OL>
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<LI> Plot mark Weight</LI>
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<LI> Plot mark Color</LI>
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<LI> Plot mark Fill color</LI>
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</OL>
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<P> The exact meaning of the parameters will of course depend on the
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type of plot marks being used.</P>
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<P> The callback must be a global function and is installed with a call
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to <A href="../ref/PlotMark.html#_PLOTMARK_SETCALLBACK">
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PlotMark::SetCallback()</A></P>
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<P> So for example to install a callback that changes the fill color for
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all marks with a (Y) value higher than 90 you could add the lines</P>
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<P>
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<!--?
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$t='function MarkCallback($aVal) {
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if( $aVal -->
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90) $fcolor="red" else $fcolor=""; return array("","",$fcolor); } ...
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$plot->mark->SetCallback("MarkCallback"); ...'; ShowCodeSnippet($t); ?></P>
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<P> As you can see in the above example we have left some of the return
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values blank. Doing this will just ignore any change of these value and
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use the global settings for the plotmarks.</P>
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<P> If you also let the (Y) value affect the size of the plot marks you
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can get what is sometimes known as a "balloon plot". The example below
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is basically a scatter plot that uses filled circles to mark the
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points. A format callback is then used to change the color and size
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depending on the Y-value for each plot.</P>
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<P><DIV class="example">
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<BR> <A href="exframes/frame_balloonex1.html" target="blank"><IMG border="0"
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HEIGHT="300" src="img/balloonex1.png" WIDTH="400"></A>
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<BR><B>Figure 92:</B> Creating a balloon plot by using plot mark
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callback function <A href="exframes/frame_balloonex1.html" target="blank">
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[src]</A>
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<P></P>
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</DIV></P>
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<P></P>
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