JIRA has graphs showing the evolution of created/closed issues:
Does GitHub have the same kind of graph?
If not, is it available on a third-party website, or generatable via a script?
GitHub now offers similar information using the Pulse feature. By clicking on the left-most heartbeat icon of the repository you desire and setting the desired timeframe, you can see the number of issues created and resolved, though not in a graph manner, but in the form of a bar.
Pulse data is also available only for recent data - longest period that may be selected is one month.
No the graph feature is available only for the commit activities not for issue tracking in GIT...
I also searched for the same feature i didnt get anything... But it is not a big deal to create the graphs for that... if you are good in web scripting you can use GIT API to get the issue and date and google chart api to create chart with those data