Could I suggest the following extension to plot.formula:
plot(cbind(y1,y2) ~ x, ...)
should plot (y1 against x) and (y2 against x) on the same plot.
Trellis - er - lattice graphics uses a similar approach, but you use '+', and give another argument, allow.multiple:
xyplot(y1+y2~x,data=xyy,allow.multiple=T)
I'm no great user of lattice graphics, so I leave it to someone else to tell us how to join the dots with lines.
The cbind() approach has a precedent in glm(cbind(y,n)~x,family=binomial)
Although I've always found plotting with formulas a bit ugly, since the 'x' and 'y' are in reverse alphabetic order and I could be plotting things that conceptually have no formulaic relationship - x and y geographic coordinates, for instance. In a modelling context you can read the ~ as 'depends on', but not necessarily in a plot!
Baz
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