Currently in Lightning Report Builder you can't display Report Column Groups in a Bar Chart.
This is especially problematic if you are on a mature org in Classic UX with a lot Charts and Dashboards that reference Matrix Columns and want to migrate to Lightning. Because if you currently have a Report and Chart like this
And you migrate to Lightning, you'll get something like this
So in order to get the Chart you want you need to "swap" your Row and Column groups like this
*** Spoiler Alert: This will break any Report Summary Formulas that you have ***
So your options are:
1. Rebuild every Matrix Report and Chart (and Report Summary Formula) on your org!
2. Leave the Report "as is" and tell your users they can't have the Chart they want.
3. Create 2 versions of the same Report so they can have the Report and Chart (but not together)
4. Remain on Classic UX
5. Vote for this Idea... a LOT!!!