How to add additional data element to stacked bar chart?

[pacificbeavs]

I have a stacked bar chart, which shows drug spend by month.  The x-axis is year/month, the y-axis is price, and the bars consist of the different drugs.  However, I need to know which person used which drug.  I tried adding a member identifier to the different "additional field" options, however I still can't see the member information when I hover over the section of the bar.

Any ideas as to how I can keep my visualization the same, but also see the member for each drug/month when I hover?  Thanks!