No blank cells in the source data range!
Excel version 2003

I should perhaps add that the offending error message suggests the following advise:

- Enter only values that are positive in the cells used by the chart
- Unselect the logarithmic scale

I also changed the formula to ABS(LOG(1-$A1)/LOG(1+$I$1)), thereby eliminating the leading minus sign. Same result.

Apparently, Excel is suspicious of the ABS(LOG(1-$A1)) part of the formula. If you substitute this test formula for the original one, the message persists.

The 1-$A1 part could theoretically result in a negative argument for the LOG function, but the actual data don't as the values in the $A column are between 0 and 0.999

In my opinion the resulting message is misplaced, not to say an error (or should I say bug in this case?)

Thanks Jon for your continuing support.