cmd/perfdash: plot all buckets
cmd/perfdash: plot all buckets

This plots all buckets on top of each other, using the
strongest color (opaque red) for the most recent bucket,
and fading away (to nearly transparent red) for the
oldest bucket. The current, incomplete bucket is plotted
in semi-transparent gray.

Closes [unavailable]

chain/chain,chain/chain,chain/chain,chain/chain,chain/chain,chain/chain