Simultaneously high swap and unused memory on Ubuntu server

I have an Ubuntu 10.04 web server in the cloud, with 1 GB of RAM. Here's a memory chart from Munin:

but I'm having trouble making sense of it: on the one hand, unused memory is very high; but swap memory is simultaneously very high, and "committed" memory is way over what's actually available.

Isn't swap supposed to be used only if there's no real memory left? Is this a normal memory usage graph, or is something probably wrong here?

Isn't swap supposed to be used only if there's no real memory left?

That's incorrect. When the kernel sees memory pages that have gone unused for a long time, it'll proactively swap them out (even if there is plenty of other free RAM), making that RAM space available for disk cache.