I've found a lot of posts about problems doing various actions across
worksheets but can't find any general information on this. Does anyone
know of a resource or could anyone provide a summary of what is and is
not possible to do across worksheets? Specifically:

Calculations across worksheets
Charting across worksheets (inc series across worksheets)

Effectively I have 3D data that I want to plot:

Individual expenses
Categories
Months

I want to list individual expenses in a particular category in the
column for that category. I then want to be able to compare that
column with the same column for different months (on different
worksheets). This seems like an elementary thing for Excel to do but
the posts I've read suggest otherwise. Could someone help me please?!