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  28  <H2><A NAME="8_17">8.17 Using callbacks for Plot marks</A></H2>
  29  <P> An interesting enhancement when using Plotmarks is the possibility
  30   to add a callback function to control the size and color of the
  31   plotmarks.</P>
  32  <P> This callback function will get called with the current Y-value (for
  33   the plotmark) as it's argument. As return value the callback function
  34   must return an array containing three (possible null) values. The
  35   values returned must be</P>
  36  <OL>
  37  <LI> Plot mark Weight</LI>
  38  <LI> Plot mark Color</LI>
  39  <LI> Plot mark Fill color</LI>
  40  </OL>
  41  <P> The exact meaning of the parameters will of course depend on the
  42   type of plot marks being used.</P>
  43  <P> The callback must be a global function and is installed with a call
  44   to <A href="../ref/PlotMark.html#_PLOTMARK_SETCALLBACK">
  45   PlotMark::SetCallback()</A></P>
  46  <P> So for example to install a callback that changes the fill color for
  47   all marks with a (Y) value higher than 90 you could add the lines</P>
  48  <P>
  49  <!--?
  50  $t='function MarkCallback($aVal) {
  51      if( $aVal -->
  52   90) $fcolor=&quot;red&quot; else $fcolor=&quot;&quot;; return array(&quot;&quot;,&quot;&quot;,$fcolor); } ...
  53   $plot-&gt;mark-&gt;SetCallback(&quot;MarkCallback&quot;); ...'; ShowCodeSnippet($t); ?&gt;</P>
  54  <P> As you can see in the above example we have left some of the return
  55   values blank. Doing this will just ignore any change of these value and
  56   use the global settings for the plotmarks.</P>
  57  <P> If you also let the (Y) value affect the size of the plot marks you
  58   can get what is sometimes known as a &quot;balloon plot&quot;. The example below
  59   is basically a scatter plot that uses filled circles to mark the
  60   points. A format callback is then used to change the color and size
  61   depending on the Y-value for each plot.</P>
  62  <P><DIV class="example">
  63  <BR> <A href="exframes/frame_balloonex1.html" target="blank"><IMG border="0"
  64  HEIGHT="300"  src="img/balloonex1.png" WIDTH="400"></A>
  65  <BR><B>Figure 92:</B> Creating a balloon plot by using plot mark
  66   callback function <A href="exframes/frame_balloonex1.html" target="blank">
  67  [src]</A>&nbsp;
  68  <P></P>
  69  </DIV></P>
  70  <P></P>
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