Using a feature called Quick Table Calculations, this is actually pretty easy to do!
I started by building a view of your Count Distinct of patients, broken down by Charge Items and Physician. This is the view below.
Now, you'll want to navigate to the columns shelf and right click on your CNTD green pill. This gives you a menu of options. You'll select Quick Table Calculation and then, Percent of Total.
Tableau is computing that percent of total calculation for you, without you having to write anything! Hurrah! But you'll want to change how it's being computed to get the correct result for your use case. You're interested in the percentage of a physicians patients that used a particular item out of all of the physicians patients. You'll calculate this using Pane Down, which tells Tableau, (in the context of this viz), that we want this calculation to rerun for each physician. In this case, you're breaking down your bar charts by physician so each new physician is a new pane.
And that's it! I added some fake cost information to the data set, to show how you can use color to display additional information. Then you can easily compare multiple metrics. I'm attaching a workbook for your use.
Percent of Total Items.twbx 23.9 KB