[gradsusr] How to plot data on a base image?

[Sherry Fu]

Hi,

I have a base image which is 720x960 pixels.
If I want to plot my data such as flow inside a boundary, how can I do 
it?
The lower right boundary pixel coordinate is (120,200) and upper left 
is (604, 788).
I tried to use:
set parea xmin xmax ymin ymax
and
set vpage xmin xmax ymin ymax
However I couldn't figure out the inches on the page correlated to the 
pixel value.
Is there other ways to do it?
Thank you.