How to combine Pre-defined Values in Custom Chart

Is it possible to apply some basic math operations on the key for build or project-level charts or is their some other way to achive this?
I want a total, not an average.

i.e. I would like a basic chart of my total build times:

"BuildCheckoutTime + BuildDuration" title="build duration ( check-out and build)" buildTypeId="bt7"/>
  <valueType key="BuildCheckoutTime + BuildDuration + BuildArtifactsPublishingTime" title="Total build duration" buildTypeId="bt7"/>

thanks in advance.