Hi Charles,
First create a formula which will convert Feet into Miles, (I don't know the exact number but below is the formula)
{Feet}/2500
(approximate value I have given, pl. give correct number)
Now right click on the graph and go in 'Chart Expert'--Data-- change the ' Show values field' with the above new formula.
Now this will convert your chart numeric values.
Thanks,
Sastry