On my Piwik installation (0.2.24) the Visit Time graphs are really goning crazy lately.
At the moment (~15:30) the scale starts 1h which is obviously incorrect. Then it immediately jumps on to 6h which is because there was no visitor from 2h-5h I suppose. Then everthing is correct from 6h 14h. This is followed by the hours that have no visitor (yet): 0h, 2h-5h and 15h-23h.
I also attached a screenshot.