So this is pretty cool, I've installed luci-app-statistics and collectd-mod-exec packages and found a not too difficult way to get realtime graphs of data on the Yun. The exec plugin runs a specified script or command which sends measurement data to standard out in a special format. Luci statistics adds a new tab in luci that shows charts for data logged by collectd, including custom data logged by the exec plugin. The slick thing is that all of this is configured through the luci interface.
The only thing missing out of the box is some lua code to tell luci how to graph the data read by the exec plugin, and a script to grab data from the ATmega and send it to collectd using the exec plugin. I'm working on something general for both so you could output sensor values to the console in a sketch (with a simple format like "Name:value", i.e. "Light sensor:999") and have it show up in the statistics charts automatically. Here's a quick look using randomly generated data right now:
Should have it working next week and will post more details.