Maybe I've not explained myself. If you do a graph twoway command:
graph tw (line y x) (line y2 x) ... (line y10 x),
where for certain reasons you cannot -by- the graph across 10 groups,
each of those line plots constitutes a serset in Stata (set of data
held in memory by Stata to use in a graph).
Now, ANYTIME you have > 99 sersets you will break Stata's abilities to
graph what you want. So in the graph command above, if you tried to
combine 9 graphs you'd be fine. But if you try to combine 10, Stata
fails.
To me this seems like a very low limit of sersets for graph combine. A
full page graphic with 20, 10-line graphs is completely readable when
saved to .eps format and printed at high resoultion (e.g., publication
quality). Yet I cannot do this within Stata because of the serset
limitation of 99.
