<p>I'm using Excel 2007 to create a log-scale chart of numbers (specifically the Zimbabwean dollar exchange rate) over time. I'm using an x-y scatterplot and noticing one odd quirk.</p>

<p>The range of y values (numbers) spans a factor of about 10^30. On every chart I make using this data, half the gridlines are missing. Specifically, only the gridlines corresponding to the largest values show up. In fact, regardless of the total range only the top factor of 10^13 or so have gridlines. This is not dependent on the log base.</p>

<p>Am I doing something wrong? Is this a known bug? I can't find any references to this issue on google or microsoft's bug reports. </p>

I'm having the same problem, it's definitely a bug.
Try a sequence 1, 10, 100, 1e+12, 1e+30 vs 0..4 and plot x,y scatter, and clearly the scale grid is messed-up even in linear, and in log is the behaviour you described.
My workaround was to make a transformation of the values and depict them scaled down (by a Million factor). That way the data the graph is handling is never above 10e9 (the value I started to hit issues).
So, my suggestion is: graph a Log version of the data (and clearly make a legend for it)