Troff "grap" question

[Erik Ekkel]

I want to use the troff "grap" preprocessor to plot a graph. The problem is
that the input has as x-coordinate weeknumbers. So the range of the
x-axis is e.g. 8801:8852, 8901:8952, ..........

How can i tell "grap" that there is a gap in the x-axis; that week 8901 is the
successor of week 8852 and NOT 8900 !
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