Performance Statistics say "nan B" for hour, day, week - month and year ok?

Hi all - Subject sort-of says it all .. for all of my mounted drives, Performance Statistics say "nan B" for the hour, day and week views, but month and year have data and display okay.
Is there something I can do to trigger proper (re)generation?

I did click Refresh and it updates the month and year graphs, but all others say "nan B."
Timezone is ok, and no other errors I can see in the logs.
It's nice to have some info, but if I can get full graphs on each panel that would be even better.

Thanks!