cryptic error message from "plot"

Hi,

I am reporting this because from the error message it is
not obvious what the problem is and it took a while for me
to figure it out.

I kept getting these errors:

 plot,[1,2,3],[4,2,3]
% PLOT: Data coordinate system not established.

until I figured out that my !p.position was invalid

IDL> print,!p.position
      1.40000     0.300000      1.70000     0.800000

Of course the solution is to properly set the position,
and is obvious in hindsight, but the error message
is a bit misleading...

Cheers,
Paolo