Thanks.

I think my analysis is correct, excel formulae cannot return empty (Hello MSoft, please add an empty() function!), so one has to use VBA.

I guess there is actually some logic to this, in that making the cell empty, would also erase the formula!

One solution is to have a second copied version of the data, but only copy if blank, and clear the copy destinations first (requires a macro or VBA). What a messy situation for a simple goal!

A better solution is for MSoft to allow for other missing values in graphs.