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(gnuplot.info)grid_data

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 The 3D routines are designed for points in a grid format, with one sample,
 datapoint, at each mesh intersection; the datapoints may originate from
 either evaluating a function, see `set isosamples` (Note: isosamples ), or
 reading a datafile, see `splot datafile` (Note: data-file ).  The term
 "isoline" is applied to the mesh lines for both functions and data.  Note that
 the mesh need not be rectangular in x and y, as it may be parameterized in u
 and v, see `set isosamples`.

 However, `gnuplot` (Note: gnuplot ) does not require that format.  In the
 case of functions, 'samples' need not be equal to 'isosamples', i.e., not
 every x-isoline sample need intersect a y-isoline. In the case of data files,
 if there are an equal number of scattered data points in each datablock, then
 "isolines" will connect the points in a datablock, and "cross-isolines" will
 connect the corresponding points in each datablock to generate a
 "surface".  In either case, contour and hidden3d modes may give different
 plots than if the points were in the intended format.  Scattered data can be
 converted to a {different} grid format with `set dgrid3d` (Note: dgrid3d ).

 The contour code tests for z intensity along a line between a point on a
 y-isoline and the corresponding point in the next y-isoline.  Thus a `splot`
 (Note: splot ) contour of a surface with samples on the x-isolines that do
 not coincide with a y-isoline intersection will ignore such samples. Try:
        set xrange [-pi/2:pi/2]; set yrange [-pi/2:pi/2]        set function
 style lp        set contour        set isosamples 10,10; set samples 10,10;
        splot cos(x)*cos(y)        set samples 4,10; replot        set samples
 10,4; replot