In Stacked column chart, Consider,

I have 3 main categories, i.e. Product1, Product2, Product3..

Each Product has 2 sub categories i.e. Open Bugs, Resolved Bugs..

Bugs are raised by Robert and David are the our Legends..

Now..I am suggesting to provide an option Like drop - down (May be multi select) which will contain all sub categories i.e. Open Bugs and Resolved Bugs..

So if user selects any one of them..charts will display all data for that selected value..in above example if user selects Open Bugs option the all product will hide data related to Resolved bugs and will show only Open bugs data in charts..

This will help us where categories and sub categories are more in numbers..

This can be applicable for both categories and sub categories..