Re: [Gnuplot-info] SurfacePlotDataMatrixProducesAxesNoSurface

[Geoff K. <ge...@ka...>]

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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