I think this is related to the tasks plugin (I'm running version 2.2)

Actions to reproduce:
1) Make sure your build has the tasks plugin enabled
2) Start a build
3) Go to the build status page (http://<server>/hudson/job/<jobName>/

The "Open Tasks Trend" graph is not displayed (I'm sure thats because the build
is occurring right now, but it should be smarter. Only display the builds that
have occurred. It should try to display data for a build that is currently
running).

I have an image that I'll attach, which should make more sense.