[lamchopz]

actually, no. it's the convention of data representation in science.

you're not meant to represent two sets of data of different units on the same graph/chart because it does not show any comparison (I take it you're trying to show some relation between the two series).

if you want to demonstrate the trend, you might want to put the two graphs one after the other, or side by side.

but as far as the technical details of putting two vertical axes in Excel go, I have no idea apart from the fattening of one column series or ... sketching by hand.