I just noticed these on linode dashboard. Seems some unusual things happened? Any ideas? How to check? Thanks.

Looks like someone took your graphs, and then drew large freehand red circles on them.

haha...

_________________
http://www.aobo.com.au

The blank area around 17:30 could have been due to your server going down. You can easily check this with the uptime command, or simply by looking at another area of the dashboard. If your server wasn't down during that time, it could also have been a benign and temporary glitch in the host machine's graph-drawing program. (One of the monkeys that sit on top of server racks and draw all those graphs might have needed to go take a dump. They're trained not to defecate in the vicinity of sensitive electronics.)

The I/O spike around 00:00 looks like a very large amount of data being crunched somehow. Since swap usage remained low, it's unlikely that you ran out of RAM. It might have been a cron job that zipped up several gigs of website files, log files, database dumps, etc. You also have a few smaller I/O spikes, all very close to the hour mark. Check your cron jobs. (Or it could have been one of those monkeys rummaging inside your server looking for homo sapiens porn.)