Support multiple expressions in pie charts

[grobie]

With the move to use less labels, but more separate metrics it's seldom that the data I want to display can be retrieved with a single expression. For example: the best practices stated in #prometheus on IRC say one should rather use separate metrics to track success and failure response rates (e.g. requests_success vs. requests_failures).

While it's possible to create rules to merge separate metrics back together, it would be great if a pie chart would more behave like normal graphs and allow the result of multiple expressions to be plotted.

[stuartnelson3]

so you want to define multiple expressions, and have them plotted in a pie chart against their summed total?

i assume you also want to support expressions that return multiple values and single values being mixed?