If I'm reading your post correctly, take a look at these values in the docs for GD::Graph

x_label_skip, y_label_skip

Print every x_label_skipth number under the tick on the
x axis, and every y_label_skipth number next to the tick
on the y axis. Default: 1 for both.
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Update:
I misread this question, but the settings I pointed out above are still useful for some graphing ops, so I'll leave the post intact.

ryddler