Hi d_t_a,
There are a few ways to produce a runtime graph, and they all involve a way to measure the light. An android phone that has a light sensor in it can be used. Another way is by using a Luxmeter such as this one:
http://a.co/dA3Firy
or similar like this one:
and recording the results, saving the results as CSV and using that to produce a graph in excel or a similar spreadsheet type program.
I use a setup similar to the following to produce mine.
Zac Wilson (he’s seen on here and a few other places thanks Zac) made a nice android utility that I frequently use.
His app is called ceilingbounce and can be found over on Github:
There’s a forum post about it here: Introducing Ceilingbounce - flashlight testing and runtime graphs for Android
There’s also quite a few posts about how to measure light and integrating spheres and a whole lot of other great stuff if you do a little searching here on the forum.
Good luck and thanks for your interest, learning new stuff is fun.