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...