[R] split a y-axis to show data on different scales

[hadley wickham]

> Spencer Graves:
> > but I believe there are times when scale breaks are
> > appropriate, but the display should make this nonstandard transition
> > very clear;
>
> ... in which case you are close to having two graphs
> sharing an x-axis and therefore saving on ink (yay!).

If your main concern is saving ink, I suggest using white points ;)

Hadley