Zero visits in graph but visits are in Visitor Log

Yesterday I had plenty of visits to one of my sites and they all show in the log, but I just noticed the graphs are showing zero visits for that day only. This only happened to one of the sites I monitor.

It’s just rolled over midnight here and I can see today’s visits are showing correctly on the graphs so far.

Anyone else seen this happen?

EDIT: attached a pic

can you delete the tables piwik_archive_numeric and _blob for this month and try again?