How to debug plotting error

[VBAhack]

Using VBA modules in a data analysis application to compute statistical results that I update to an Excel worksheet with existing plots (i.e. the plots aren't created each time but automatically update because the underlying data changes). Several plots use log scale. None of the result are negative and I've taken great pains to make sure none of the results are zero by making sure any result is at least E-17.

From time to time get the annoying dialog box with the message that "Negative or zero values cannot be plotted on log charts" but cannot for the life of me find any result that is zero or negative. Since Excel found something it didn't like, is there any way to further investigate exactly what it doesn't like? It is sort of like going to a doctor with a pain but not revealing the location of the pain.