The easiest choice would probably be to format the y axis as a "logarithmic axis" -- a simple checkbox in the format axis dialog.

Just about any other approach I can see would be some variation of Jon Peltier's "arbitrary axis" technique. An example of this technique is here: http://peltiertech.com/Excel/Charts/...lityChart.html Note how he creates a "dummy series" to serve as the axis, then labels that series (using Rob Bovey's XY chart labeler add-in), then hides Excel's default axis, to create an arbitrary axis.