dashed lines using plot

[Alec Joubert <alec@xxxxxxxxxxxxxxxxxx>]

Hello All

I am trying to control the use of dashed or coloured lines using plot.
I know that it's possible to use the following command sequence to control
line colours and thicknesses:

plot/set_up .....
ppl pen ? []
ppl plot

This appears to have the same kind of result as using plot/line=n, where 
n specifies a pen colour or thickness. The manual states, though, that
line styles are device-dependent and may appear as either colours or
dashed lines. 

I'm plotting more than one curve on a graph, using:

plot/set_up x,y