Page count graphs may be cached into the following day

[windymilla]

If viewing a graph near the start of the server day, the user may get a cached version that was calculated the previous day.

In this graph, for example, it can then look as though no pages have been done today.

Maybe some graphs' cached version should be cleared at midnight.

[cpeel]

The way this is suppose to be working is that the daily pages graphs (the first two on that page) are cached until midnight server time: https://github.com/DistributedProofreaders/dproofreaders/blob/master/stats/pages_proofed_graphs.php#L23

The data for the graphs is pulled from the tally snapshots that are captured at 1 minute after midnight (controlled by crontab), so its possible we have a bit of a race gap where someone could request the page between midnight and 1 minute after midnight and we're caching that. Or we have a bug in how that "tomorrow" expire time is calculated.