help making a y-axis that makes sense for fractions

Hello! I'm using excel 2010 and I have data that is graphed as [product]/[precursor].

Currently the y-axis looks like this:

3----|
2----|
1----|
0----|______________

Which is okay for when the product is bigger than the precursor. However, when the precursor is bigger, it's going to be a fraction and all the datapoints are cluttered up between 1 and zero. So what I would like to do is have the y-axis look like this:

4----|
2----|
1----|
.5---|
.25--|________________

But I can't find any options to manually specify what each y-axis line should be. All the options in "format axis" are way too general: all I can do is set the bounds and the units, which doesn't work for both sides of "1" in the way that I wanted to.

Please help!

Originally Posted by MrShorty

Another thought. Based on your crude "text" graphs, what you are showing is a logarithmic y axis (base 2). Base 2 means that each of your major gridlines/tick marks/divisions is a power of 2 (2^-2,2^-1,2^0,2^1,...). If you and your users can handle a logarithmic axis, this might be the easiest way to get what you want. Logarithmic axis should be an option in the format axis dialog.

Brilliant and simple. Thank you much.