Ongoing line or scatter chart, don't want bar to drop to zero on blanks

I'm using Excel 2010, and I can't seem to figure this out. This thread has almost the same issue, but the solution posted doesn't seem applicable to 2010.

I'm working on a workbook for tracking debt. It has 15 sheets. Instructions, Charts (for showing debt change over time), "Invisible" (this is where my chart data is, I plan to hide it), and 12 month sheets. On the Invisible sheet, for example, I have my data set up. I'm pulling the data from other sheets. In the screenshot, I'm using =Month12!P48 as the formula to pull my data from the other sheets. But, the user hasn't entered data yet for month 12, so the line chart drops to zero there. I want it to just stop and do nothing, until there is data in that cell. (I would prefer to use a line chart vs a scatter chart for this, but I have the same problem in both)

data.jpg
chart.jpg

try
=if(Month12!P48=0,na(),Month12!P48)