[idvusers] Simple difference a-b with data from two netcdf files


I am a new idv user, trying to do something that apparently should be easy.
I am loading two netcdf files with multiple 2D fields. I simply want to subtract one field in one file from another field in another file. I load and plot both fields with Color-shaded Plan View, that works fine.

I then use Data->Formulas->Miscellaneous->Simple difference a-b
and choose Colour-Shaded Plan View again. Now I get a window where I can choose a and b. The problem is that I can only choose the files, not the fields in the files. At the same time, idv seems to get very busy. I thought it might try to load all the data again, but the files are not very big, and even if I wait a long time, nothing happens. So when I then click 'ok' I get

An error has occurred:
Creating display: Color-Shaded Plan View
No data selected

How do I get around this problem?

-Hilde Fagerli-