NR Bar Chart - 2 Data Set (Neg/Pos)

I'm working on a Chart where I need to show 2 data set - Negative (red) and Positive (green) in a bar chart. I created an Ifthen formula which looks for less than/greater than in 2 new columns,so I can plot them separately but I was wondering is there a way to write the ifthen formula in an NR report to return Blank instead of 0, I tried but I'm getting an error?

thanks and regards

dale