I just asked Trevor what was going on. Apparently we turned off the restart-HN process back when we were hacked a few days ago, and we forgot to turn it back on. Things should be back to normal now.
It looks like it's just up or down on the y-axis. RRDTool is often used to generate more complex graphs, so something this simple isn't always the most intuitive.
The monitor checks HN once per minute and records a 0 for up or a 1 for down in the database.
RRDTool then averages these values out nicely so that, for example, when the graph is at weekday-resolution and you see a 50% filled bar spanning one day, then HN was down half of that day.