cmd/perfdash: show more range from histogram
cmd/perfdash: show more range from histogram

The "significant figures" parameter controls how many of significant
figures of the recorded value the histogram can represent, it's not how
many digits of quantile we can evaluate. So it is meaningful to go to
p99.99999 even with only 3 digits of precision.

Closes [unavailable]

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