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<b class="current">Contour Maps</b>
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<div><h1 id="sec:contourmaps">1.13 Contour Maps</h1>
<p>Contour maps are similar to colour maps, but instead of colouring the whole <img src="images/img-0188.png" alt="$(x,y)$" style="vertical-align:-4px;
width:40px;
height:18px" class="math gen" /> plane, lines are drawn to indicate paths of constant <img src="images/img-0535.png" alt="$c(x,y)$" style="vertical-align:-4px;
width:49px;
height:18px" class="math gen" />. The number of contours drawn, and the values <img src="images/img-0040.png" alt="$c$" style="vertical-align:0px;
width:8px;
height:8px" class="math gen" /> that they correspond to, is set using the <tt class="tt">set contour</tt> command<a name="a0000000854" id="a0000000854"></a>, which has the following syntax: </p><pre>
set contours [ ( <number> |
\( <value> {, <value>} \) ) ]
[ (label | nolabel) ]
</pre><p>If <tt class="tt"><number></tt> is specified, as in the example </p><pre>
set contours 8
</pre><p> then the specified number of contours are drawn at evenly spaced intervals. Whether the contours are linearly or logarithmically spaced can be changed using the commands </p><pre>
set logscale c1
set linearscale c1
</pre><p> By default, the range of values spanned by the contours is automatically scales to the range of the data provided. However, it may also be set manually using the <tt class="tt">set c1range</tt> command<a name="a0000000855" id="a0000000855"></a> as in the example </p><pre>
set c1range [0:10]
</pre><p> The default autoscaling behaviour can be restored using the command </p><pre>
set c1range [*:*]
</pre><p>Alternatively, an explicit list of the values of <img src="images/img-0040.png" alt="$c$" style="vertical-align:0px;
width:8px;
height:8px" class="math gen" /> for which contours should be drawn may be specified to the <tt class="tt">set contour</tt> command<a name="a0000000856" id="a0000000856"></a> as a ()-bracketed comma-separated list. </p><p>If the option <tt class="tt">label</tt> is specified to the <tt class="tt">set contour</tt> command<a name="a0000000857" id="a0000000857"></a>, then each contour is labelled with the value of <img src="images/img-0040.png" alt="$c$" style="vertical-align:0px;
width:8px;
height:8px" class="math gen" /> that it corresponds to. If the option <tt class="tt">nolabel</tt> is specified, then the contours are left unlabelled. </p><p>In the following example, a contour map is overlaid on top of a colour map of the function <img src="images/img-0536.png" alt="$x^3/20+y^2$" style="vertical-align:-5px;
width:84px;
height:21px" class="math gen" />: </p><p> <tt class="tt">set nokey</tt><br /><tt class="tt">set size 8 square</tt><br /><tt class="tt">plot [-10:10][-10:10] x**3/20+y**2 with colourmap, <img src="images/img-0006.png" alt="$\backslash $" style="vertical-align:-5px;
width:7px;
height:18px" class="math gen" /></tt><br /><tt class="tt">x**3/20+y**2 with contours col green lw 2 lt 1</tt> </p><p> <center>
<img src="images/img-0538.png" alt="\includegraphics[width=10cm]{examples/eps/ex_contourmap}" style="width:10cm" /></center> </p><p>The <tt class="tt">contourmap</tt> plot style differs from other plot styles in that it is not permitted to take expressions such as <tt class="tt">$2+1</tt> for style modifiers such as <tt class="tt">linetype</tt> (see Section <a href="sec-with_modifier.html">1.1</a>) which use additional columns of input data to plot different points in different styles. However, the variable <tt class="tt">c1</tt> may be used in such expressions to define different styles for different contours: </p><p><table cellspacing="0" class="tabular">
<tr>
<td style="text-align:left"><p><img src="images/img-0029.png" alt="\includegraphics{cross}" style="width:20px; height:24px" />
</p></td>
<td style="text-align:left"><p><div style="width:0.0pt" class="minipage"><tt class="tt"> plot ’datafile’ with contourmap linetype $5 </tt></div></p></td>
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</table> </p><p><table cellspacing="0" class="tabular">
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<td style="text-align:left"><p><img src="images/img-0030.png" alt="\includegraphics{tick}" style="width:19px; height:17px" />
</p></td>
<td style="text-align:left"><p><div style="width:0.0pt" class="minipage"><tt class="tt"> plot ’datafile’ with contourmap linetype c1/10 </tt></div></p></td>
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