Improve the demo instance's graphs

[thislooksfun]

Is there an existing issue outlining your problem?

• I have searched the existing issues and they do not solve my problem.

Describe your issue.

Currently the demo instance has no historical data when it is first started (which is expected, since it is a brand new Home Assistant instance), but this means that it is really hard to use it to get a decent screenshot of a graph view. It would be nice if we could inject some more interesting looking data into the history of the graphed values during startup.