When gnuplot generates coordinates, it uses the spacing described by these
keywords. For example dx=10 dy=20 would mean space samples along the
x dimension by 10 and space samples along the y dimension by 20. dy cannot
appear if dx does not appear. Similarly, dz cannot appear if dy does not
appear. If the underlying dimensions are greater than the keywords specified,
the spacing of the highest dimension given is extended to the other dimensions.
For example, if an image is being read from a file and only dx=3.5 is given
gnuplot uses a delta x and delta y of 3.5.
The following keywords also apply only when generating coordinates. However
they may also be used with matrix binary files.
