Subtract two values with graphite possible?

Hello, i have following problem:
I already show the Windows Server File Share Quota Status on he Grafana Dashboard. I use the pie chart plugin from the plugin library.
The workflow how grafana gets the values are following: icinga2 (check_drivesize via NSClient++) -> performance-data -> graphite -> grafana.
From the check_drive plugin i get only the used and the total value. This means i use two metric lines. One is total and the other is used.
The problem here is that to show the graph correctly i would need the “used” and “free” value but i dont have the free one right now.
So my thinking was that a workaround could be that i subtract used from the total value. Is this possible in grafana?

Edit: Same problem like here: https://community.grafana.com/t/disk-usage-as-percentage-of-used-disk/1499/3

Thank you!

There is a graphite function called substract i think

I could not find a function called “substract” do you have a link or example for me?

It’s called diffSeries