Expressions on sources that don't support them (aka stackdriver)

I’ve enjoyed using grafana with prometheus before and so when faced with working with stackdriver, went “ugh”.

Then I found the stackdriver source for grafana and thought my problems were solved.

But it appears that some of the things I liked about the Prometheus/Grafana combo were from Prometheus, not Grafana.

Specifically, I was able to do math on sources; stuff like take the ratio of two metrics, or the difference between two metrics, et cetera.

Is there any way to create expressions on metrics from sources that don’t support expressions? Or more specifically to do them on stackdriver sources?