I believe you'll need to graph on a formula instead of the field, and the formula should look something like this:
If {table.graphField} = "" //OR If IsNull({table.field})Then
"N/A"
Else
{table.graphField}
I have not done this before, so my syntax may be haywire, but I believe this will put you on the right track... Let me know if I am way off base! -rodbac