Excel Charting: Secondary axis with categorical value

I am drawing a series of chart with x-axis as time and y-axis as deformation (say of a slab). Chart type is x-y line. I got a chart with x axis as equal division of time. Now I want a secondary x axis which gives the categorical value, the days position (provided in the input) on which the deformation measeured.
In short, I want a secondary x axis which shows the categorical value on the primary value axis??

Any one have the answer, plz post.

Raj