Hi,
here is a solution:

Assume the cells used for the chart has the following formula:
= C3*D3

Then change this into =if(C3=0,NA(),;C3*D3)

This will cause a "missing"-sign in the december cell, and the graph will not show this value at all.

I hope I got it right above with translation of commands from my Swedish Excel version into English.

Kind regards
Bjorn