plot line caps: mac vs linux behavior

I'm making plots with the linewidth set to some large value (~10) in order to visualize good-time-intervals for astronomical observations. Under GNU/Linux, the plots behave as I'd like them to (no caps on the ends), but under Mac OSX, the the lines are extended by linewidth/2 on each end (the dreaded caps).

How do I get rid of the caps in Mac? When I have a lot of closely spaced intervals, I end up with a single line, all run together.

Peter

[Jan]

Can you post a picture?