Drill Down Line Chart Over Time

I have a line chart with the X Axis showing my year, and the Legend showing my Product Category (i.e. I have 10 different Products for 10 different lines on my chart). The value shown is revenue.

I want to be able to drill down into my Sub Product Category so I have a Hirearchy of Product Code and Sub Product Code

However, when I add this to the legend box the Sub Product Code dissapears, and when I click on it to add it from the Fields box it shows up in the Axis box instead. Thus when I drill down I only get a scatter plot of Sub Product Codes at that time period, instead of a graph of the Sub Product Codes for that Porudct Code over time

What am I not understanding about drill down?