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update 2004/12/17
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3-Dim Plot with a Color-Map (pm3d)

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How do I draw a colored 3D figure ?

[ver.4] ONLY !

Gnuplot draws 3D figures with lines and / or points, in addition, ver.3.8 or 4.0 allows you to draw a color-mapped 3D figure by setting pm3d . The figure is shown on your screen as well as PostScript or some image formats like PNG/JPG.

gnuplot> set xrange [-2:2]
gnuplot> set yrange [-2:2]
gnuplot> set pm3d
gnuplot> splot exp(-x*x)*exp(-y*y)
fig/sample6.7a

You can choose color or gray-scale by the terminal option of color/monochrome . The gray-scale figure can be drawn by set palette gray , but the surface grid lines are still colored in this case. The following two examples are for the PostScript terminal. 

gnuplot> set term postscript eps enhanced color
gnuplot> set output "color.eps"
gnuplot> replot
gnuplot> set term postscript eps enhanced monochrome
gnuplot> set output "mono.eps"
gnuplot> replot
fig/sample6.7b fig/sample6.7c
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I want to draw colors for contours.

[ver.4] ONLY !

A contour map of gnuplot is shown by lines. The colored contour by pm3d is something like a colored density distribution, which means, each piece of colored surface is mapped on the bottom/top plain. To show this on the bottom, add the at b option to the set pm3d command.

gnuplot> set pm3d at b
gnuplot> set ticslevel 0.8
gnuplot> set isosample 40,40
gnuplot> splot x*x*exp(-x*x)*y*y*exp(-y*y)
fig/sample6.8a

With pm3d, it is very easy to see the bottom surface in the 2D plot.

gnuplot> set pm3d map
gnuplot> splot x*x*exp(-x*x)*y*y*exp(-y*y)
fig/sample6.8b
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Pseudo 3D Bar graph

Since gnuplot cannot draw a 3D-bar graph, we need a little trick to make it. Suppose we have the following 3D data.

# X     Y    Z
  0.0   0.0  2.0
  0.0   1.0  3.5
  1.0   0.0  1.0
  1.0   1.0  3.0

The first line means Z=2 when 0< X <1, 0< Y <1, and it corresponds to the area (1) in the next figure. The data file does not contain maximal values. The ranges of X and Y are implicitly assumed as 1< Y <2, 1< Y <2, and the Z value is 3.0. We will make a 3D histgram like this.

fig/sample6.9

Now we expand the data with some programs/tools. See the figure above at X=0, the Y values are step function changing Y=2.0, 2.0, 3.5, and 3.5. This step function is expressed as one line in a 3D space. Same can be consider for X=1 and X=2.

# X     Y    Z
  0.0   0.0  2.0
  0.0   1.0  2.0
  0.0   1.0  3.5
  0.0   2.0  3.5

  1.0   0.0  2.0
  1.0   1.0  2.0
  1.0   1.0  3.5
  1.0   2.0  3.5

  1.0   0.0  1.0
  1.0   1.0  1.0
  1.0   1.0  3.0
  1.0   2.0  3.0

  2.0   0.0  1.0
  2.0   1.0  1.0
  2.0   1.0  3.0
  2.0   2.0  3.0

Once you prepare your data in such a way, we just depict them with lines. A program to process the data can be .... your homework. 

gnuplot> set hidden3d
gnuplot> splot "test.dat" with lines
fig/sample6.9a
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[ver.4] ONLY !

When your gnuplot is ver.4, a funcy figure can be made with pm3d. 

gnuplot> set pm3d
gnuplot> splot "test.dat" with pm3d
fig/sample6.9b
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