it all depends on the "origin" of that NULL.
In general, Qlikview gives your required result by default, with no need of any additional effort.
Let me give you an example.. take this script:
LOAD * INLINE [
Now, b is always associated with NULL, so you would expect to see a NULL in the chart...
If you make a pivot chart with
you will see only a,30
But if you add another expression
You will see also the record of b, with 0 as you required, although it is a sum of NULLs.
So, probably your NULLs are coming from a special situation (like a LEFT join.. or simply an association with missing values?...)
You should try to reproduce your error in a mini-script. This is the most difficult part. Once you have reproduced it, it's very easy to find a solution.
In alternative, if you want, you can send me the qvw and I will fix it for you