Conditional formating the #Name? error message

I'm trying to graph data that varies in length and so to avoid having blank spaces on my graph when the data is shorter I have put in the 'else' section of my If statement N/A() as recommended on previous postings.

The problem with this is that i then can't put conditional formating on the cell so i have a unsigtly #Name? error on my worksheet that I was able to hide when leaving the else section blank.

Is there a way around this that I can keep the cell blank and also not plot it on the graph when it is blank? Many thanks