But if I have enough time I will add some of the features you like (refresh interval, alarm) to your project. If not I can send you a .zip of my project (if I don't put it on github).
I would use some open-source graph-drawer-software to print the graph but I haven't read anything about it so I can't tell you something performance specific. Yes, I would save the values in a mysqlite db on the android phone (first idea).
Do it! And enjoy. I look forward to some pull requests down the road .