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<H3><A NAME="7_1_16">7.1.16 Constructing smooth line plots with Cubic
 Splines</A></H3>
<P> If you only have access to a few data points but still want a smooth
 curve between those data points JpGraph can help you achieving that by
 letting you construct cubic splines. If you never have heard of cubic
 splines before, don't worry. You only have to supply the data points
 you know tell JpGraph how many interpolated points in total you want in
 the graph. JpGraph will now take care of constructing a smooth curve
 between all you data points. The new, smooth, curve will be made up of
 exactly as many interpolated points as you have specified.</P>
<P> To construct a spline you need both the X and Y coordinates for the
 known data points.</P>
<P> You start by constructing a new <A href="">Spline</A> instance. To
 get access to the Spline class you must first remember to include the
 file &quot;jpgraph_regstat.php&quot;. You instantiate this class by calling it
 with your two known data arrays (X and Y) as follows.<DIV class="phpscript">
<CODE><FONT color="#000000"> <FONT color="#0000BB">&nbsp;$spline&nbsp;</FONT><FONT color="#007700">
=&nbsp;new&nbsp;</FONT><FONT color="#0000BB">Spline</FONT><FONT color="#007700">(</FONT><FONT
color="#0000BB">$xdata</FONT><FONT color="#007700">,</FONT><FONT color="#0000BB">
$ydata</FONT><FONT color="#007700">);</FONT><FONT color="#0000BB"></FONT>
</FONT></CODE></DIV></P>
<P>This call initializes the spline with the data points you have. These
 data points are also known as<I> Control points</I> for the spline.
 This helper class doesn't draw any line itself. Instead it is merely
 used to get a new (larger) data array which have all the interpolated
 values. You then use these new value in your plot. This way give you
 great flexibility in how you want to use this interpolated data.</P>
<P> Continuing the above line we now use the <A href="">Spline::Get()</A>
 method to get an interpolated array containing a specified number of
 points. So for example the line<DIV class="phpscript"><CODE><FONT color="#000000">
 <FONT color="#0000BB">&nbsp;</FONT><FONT color="#007700">list(</FONT><FONT color="#0000BB">
$sdatax</FONT><FONT color="#007700">,</FONT><FONT color="#0000BB">
$sdatay</FONT><FONT color="#007700">)&nbsp;=&nbsp;</FONT><FONT color="#0000BB">
$spline</FONT><FONT color="#007700">-&gt;</FONT><FONT color="#0000BB">Get</FONT><FONT
color="#007700">(</FONT><FONT color="#0000BB">50</FONT><FONT color="#007700">
);</FONT><FONT color="#0000BB"></FONT></FONT></CODE></DIV></P>
<P>Will construct the two new data arrays '$sdatax' and '$sdatay' which
 contains 50 data points. These two arrays are constructed from the
 control point we specified when we created the '$spline' object.</P>
<P> You would then use these two new data array in exactly the same way
 as you would form ordinary data vectors.</P>
<P> The following example illustrates this<DIV class="example">
<BR> <A href="exframes/frame_splineex1.html" target="blank"><IMG border="0"
HEIGHT="200"       src="img/img/img/img/img/img/splineex1.png" WIDTH="300"></A>
<BR><B>Figure 28:</B> Using spline to get a smooth curve between the
 control points. <A href="exframes/frame_splineex1.html" target="blank">
[src]</A>&nbsp;
<P></P>
</DIV></P>
<P> In order to make the example more interesting we actually use two
 plots. First a line plot to get the smooth curve and then a standard
 scatter plot which is used to illustrate where the control points are.</P>
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