[R] Line Type Specification: lty="<on><off>" but lty="<off><on>"?

[Henrik Bengtsson]

Hi,

Section 'Line Type Specification' in help(par) explains how you can do
custom line types.  For example:

plot(NA, xlim=c(0,1), ylim=c(0,1));
abline(h=1/2, col="blue", lwd=2, lty="88");

will draw a dashed line segment where the line is composed of 8 units
of "on" (blue color) and 8 units of "off" (transparent), then
repeated.

Now I'd like to draw a second red line overlapping this one, but where
the "gaps" are now red.  Technically, I think the following would
define that:

abline(h=1/2, col="red", lwd=2, lty="0880");

that is 0 "on", 8 "off", 8 "on" (red color) and 0 "off", then repeated.

However, zeros are not allowed (actually, why not?)

Any suggestions to draw one red and one blue dashed lines that, if
overlapping, the the overlapping segments will be blue, red, blue,
red, ...?

/Henrik