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Re: [Gnuplot-info] SurfacePlotDataMatrixProducesAxesNoSurface
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From: Ethan M. <eam...@gm...> - 2026-09-23 20:45:06
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You were close. The 'missing' corner of the input matrix data is ignored
but must be present (i.e. non-blank).
Here is an example showing both the data format and the addition of contour
lines on the surface. For clarity I draw the surface and the contours in
separate clauses of the splot command. This also allows me to specify
thicker lines for the contours.
On Wed, Sep 23, 2026 at 11:48 AM Geoff Kaniuk <ge...@ka...> wrote:
> Yes, today the plane was in the right position with no change in code.
>
> Regarding the matrix format, I have seen examples in "Gnuplot in Action"
> and web searches, which I copied. I have now added the Y coordinate row
> and X coordinate column - but this still left an empty surface.
>
> In my example, x and y start at the same value, so I was able to set M00
> to -19.0. But there seems to be an issue if the ranges have different
> start points: eg y in [1.0,4.0], x in [0.2,0.6]
> ?? 1.0 2.0 3.0 4.0
> 0.2 a b c d
> 0.4 b c d a
> 0.6 c d a b
>
> What value would ?? be? I tried omotting it and then get the 'data is
> not a matrix message'
>
> I was hoping to find a show command which tells me what gnuplot decides
> its zrange is, but show zrange says numeric.
>
> Why should Gnuplot think I want to do anything other than plot the
> surface showing coordinate lines?
> I noticed in my code that I had omiited 'set surface'. I have now added
> that to no effect.
>
> Is there anything I should be setting to display the surface?
>
> You, of course, are right that I do actually want to see the contour
> map. That is how I started with all the settings that are needed for
> that. When I got nowhere, I decided to simplify and just plot the
> surface, then the cotour could follow.
>
> In the meantime I really do want to get this surface map working.
>
> There must be some basic setting I am missing?
>
> Geoff
>
> On 23/09/2026 04:40, Ethan Merritt wrote:
> > The easy part first.
> >
> > The "set xyplane" command has two forms.
> > "set xyplane <foo>" interprets <foo> as some fraction of the current z-
> > range and places the plane at that fraction of the zrange (I never use
> > this and may have described it slightly wrong)
> > "set xyplane at <zval>" does exactly what it looks like - places the
> > plane at z=zval.
> >
> > Now the matrix.
> > You have provided a matrix that contains only z values, no x or y.
> > The program plots it assuming x = column_number, y = row_number.
> > See "help matrix".
> > So your surface is drawn in the area bounded by x=[0:8] and y=[0:8]
> >
> > I am assuming that you want to map it instead onto the x and y ranges
> > you have specified, but it is not clear to me (or to the program) how
> > you want to do that. Without additional information about the eventual
> > use case and data source, it's hard to given specific suggestions on how
> > to proceed.
> >
> > One option is to accept that the program will use xrange [0:8], as in
> > your example, but manually adjust the labels on the x axis:
> > set xtics ("-18.5" 0, "-13.5" 8)
> > And so on for the y axis labels. That puts the burden on you to sort
> > out what the labels should be, so it's probably not the best solution.
> >
> > Another option is to reformat the data so that it is contains x and y
> > coordinates as well as z. There are at least two different formats
> > accepted by gnuplot that should work. See "help matrix uniform" "help
> > matrix nonuniform".
> > - Ethan
> >
> > On Tue, Sep 22, 2026 at 4:08 PM Geoff Kaniuk <ge...@ka...
> > <mailto:ge...@ka...>> wrote:
> >
> > I am attempting to plot a surface from data in a file. The plot
> > produced
> > shows the correct X,Y, Z axes and key. But no surface and the base
> > plane
> > is not where I expect it to be.
> >
> > For simplicity I am testing with the data inline using:
> > splot [X_min:X_max][Y_min:Y_max] '-' matrix
> > 0.0046 -0.067 -0.1375 -0.2081 -0.2587 -0.3094 -0.3702 -0.4311
> -0.495
> > 0.097 0.026 -0.043 -0.1129 -0.1617 -0.2087 -0.2577 -0.3123
> -0.3891
> > 0.1774 0.0877 0.0226 -0.0425 -0.1011 -0.1598 -0.2069 -0.254
> -0.3127
> > 0.2578 0.1498 0.0882 0.0278 -0.0405 -0.1085 -0.1561 -0.1989
> -0.2363
> > 0.3034 0.2022 0.1448 0.0875 0.0233 -0.0408 -0.0947 -0.1486
> -0.1892
> > 0.3491 0.2587 0.2015 0.1438 0.0872 0.0285 -0.0333 -0.0951
> -0.1421
> > 0.4093 0.3214 0.256 0.1905 0.1377 0.085 0.0251 -0.0349
> -0.0886
> > 0.4695 0.3912 0.3104 0.2381 0.1883 0.1378 0.0834 0.0263
> -0.0351
> > 0.47660 0.4624 0.3798 0.2972 0.2372 0.1772 0.1293 0.0814
> 0.0225
> >
> > The preamble and splot call is contained in a gp script file like
> so:
> > [plot-xy-surface.gp <http://plot-xy-surface.gp>]
> > #!/usr/bin/gnuplot -persist
> > reset session
> > set terminal wxt 1 persist enhanced
> >
> > X_min=-18.5; X_max=-13.5; Y_min=-18.5; Y_max=-13.5;
> > Z_min=-0.6; Z_max=0.6
> >
> > unset xtics;
> > set xrange [X_min:X_max]; set xtics X_min,0.5,X_max; set xlabel "X"
> > unset ytics;
> > set yrange [Y_min:Y_max]; set ytics Y_min,0.5,Y_max; set ylabel "Y"
> > set zrange [Z_min:Z_max]; set ztics Z_min,0.1,Z_max; set zlabel "Z"
> >
> > unset key; set key outside
> >
> > set xyplane 0.0
> > set grid; set cntrparam levels 13; set isosamples 100
> > splot ....
> >
> > The terminal session is:
> > gnuplot> call "plot-xy-surface.gp <http://plot-xy-surface.gp>"
> > gnuplot> show surface
> > surface is drawn
> >
> > I have tried many variations, and have now drawn a blank! As
> mentioned
> > above there is no surface, and the base plane is set at Z_min rather
> > than 0.0 as requested!
> >
> > Is there any option that I should be turning on or indeed off?
> >
> > For reference:
> > GNUPLOT: gnuplot 5.4 patchlevel 4
> > OS: Debian 12 bookworm
> > DESKTOP Mate 1.26.0
> >
> > I have no problem using splot with functions.
> >
> > Any input much appreciated.
> >
> > --
> > Geoff
> >
> >
> >
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>
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