[R] getting lines with non-rounded caps?

[Trent Piepho]

I'm trying to get a plot with wide lines that don't have rounded end caps. 
You can see the effect quite clearly with a plot like this:

plot(0:1)
lines(c(1.5,1.5),c(0,.5),lwd=20,col="red")
lines(c(1.5,1.5),c(.5,1),lwd=20,col="green")
abline(h=0)
abline(h=0.5)
abline(h=1)

The colored lines should be between the horizontal lines, but because of the
rounded end caps, they aren't.

Under X11, if I set lty to something like "F0", the end caps are turned off. 
Unfortunately, the "0" appears to get ignored, and the line style is the same
as "FF", i.e. 16 units of drawn line, then 16 units of space.

With the postscript device, lines styles like "FF" still get rounded end caps,
unlike X11.

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