not so FAQ

• Legend
• Tics
• Label
• 2-D Plot
• 3-D Plot
  • 
    origin of Z-axis

  • 
    fine mesh

  • 
    view point

  • 
    contours

  • 
    contours on the 2D plot

  • 
    3D grid graph

  • 
    colored 3D figure
    [ver.4]

  • 
    colors for contours
    [ver.4]

  • 
    pseudo 3D bar

  • 
    change 3D colors
    [ver.4]
• Polar Coordinate
• Parametric Functions
• Data File
• After Plotting
• Miscellaneous Stuff

About 3-Dimensional Plot (No.1)

Why the origin of Z-axis is not on the XY-plane ?

In the splot , the zero-point of Z-Axis is placed above
the XY plane. To move this origin onto the XY plane, use set
ticslevel . When ticslevel=0, the zero of Z-Axis moves to on
the XY plane.

gnuplot> set ticslevel 0
gnuplot> splot (x**2)*(y**2)

I want to make a surface mesh finer.

In a 3-dimensional plot, mesh size (XY coordinate) is determined by
isosample . The smaller this number, the more rough mesh
you get. The default is 10.

If the mesh is too rough, it sometimes makes trouble when a hidden
line option set hidden3d is used. The following example
shows a 3-dimensional plot of the function z=sin(x)*cos(y) with the
isosample of 10. The top drawing is without the hidden line
removal, and the lower is with it.

To avoid this problem, make isosample larger. The next we
set it to 40. To make X and Y meshes different, try set isosample 20,40.

gnuplot> set isosample 40

How do I change a view point ?

The view point in the 3-dimensional plot is controlled
by set view command.
The default setting is,

gnuplot> show view

view is 60 rot_x, 30 rot_z, 1 scale, 1 scale_z

Initially (before rotation), your screen is parallel to
the X-Y plane and the Z-axis is perpendicular to that.

Firstly, the X-axis is rotated to 60 degrees (rot_x), then
the Z-axis slants.

Next, the new Z-axis is rotated to 30 degrees (rot_z).
This is the view point which splot sets it as default.

The set view command rotates the graph.

	set view 60,15
	set view 60,45
	set view 60,75
	set view 15,30

How do I change contours ?

Contour lines can be controlled by the set cntrparam command.

gnuplot> set contour
gnuplot> set cntrparam levels 10
gnuplot> set cntrparam levels incremental -1, 0.2, 1
gnuplot> set cntrparam levels discrete -0.2, -0.5, 0.2, 0.5

The above example shows three ways to control the contours. The
keyword levels defines the level to which contour curves
are drawn. The above, levels 10 indicates that ten
contour lines are drawn. The next, incremental defines
the initial, increment, and final values. You can also define the
height at which the contour lines are to be shown by the command
levels discrete .

A legend for contour lines is placed in the legend of the graph. To
erase the contour’s legend, use the set noclabel
command. In this case all line kinds for the contour lines become
the same, which is just next line kind to the surface plot.

The line kinds of the contours are the next to that used to the
surface. I guess there is no way to control those line kinds
arbitrarily. You have to use some external tools such as Tgif to edit
the style of the contour lines.

I want to draw only contours on the 2-dimensional plot.

When your view point is right above the XY plane, you get a
two-dimensional plot of the contours lines. Firstly set the view
point at 0,0 by set view 0,0 , and indicate set
nosurface to hide the surface.

gnuplot> set contour base
gnuplot> set nosurface
gnuplot> set view 0,0

The Y-axis goes to the right-side by the set view 0,0
command. If you want to move it to the left-side, use set
view 180,180. The Y label becomes horizontal in this
method. If you want to make it vertical, use “table” as
follows.

You can make a better two-dimensional plot of the contours by using
the table terminal. The contour lines are once written on a
data file with the set term table command, which
produces two-dimensional data of various contour lines. See
an example in our
Gallery.

gnuplot> set term table
gnuplot> set output "table.dat"
gnuplot> replot
gnuplot> set output
gnuplot> set term x11
gnuplot> plot "table.dat" using 1:2 with lines

[ver.4] ONLY !

The two-dimensional plot with a color-map can be done
easily with the pm3d terminal.