POWA demo is showing negative blocks in the overview

[gwenshap]

When I go to the demo URL and view the tpc database ( https://demo-powa.anayrat.info/server/2/database/tpc/overview/)
I see negative numbers in the blocks graph and the chart itself looks misdrawn (lines spilling out of the chart). See these screenshots:

[rjuju]

Thanks for the report!

I know that pg_stat_kcache can lead to sometimes weird results as it will see all disk access, including the one that are not related to actual buffer access. That may explain the negative number computed here, so we should fix the UI to account for that scenario in the formula that generates the number.

[rjuju]

I'm moving the issue to powa-web as it's where it probably comes from.