Speed up samples plots

[bennn]

Rendering a samples-plot is slow right now, because it uses non-optimized configuration counting.

There's a deep problem: it's hard to get the "total" number of configurations.

• For exhaustive data, the total is "all configurations"
• For approximate data, the total is the sample size

Probably need to build a hook to override.

NOTE: a bad way to override would be:

• add a num-configurations field to the performance-info struct
• set it to the sample size, sometimes

[bennn]

Please fix, it's annoying to work around this using caches