The Gnome Power Statistics (an applet for Gnome Panel) show chart (with limited options) of the battery charge (10 minutes, 2 hours, 1 day, 1 week) without the possibility to select an arbitrary interval.
After been reading the source code of this applet I’ve found that it get the data from DeviceKit (Devkit). Once again I check the source code of Devkit and I found that the data is stored in: /var/lib/upower/ (on Gentoo).
So now you can plot this data with another tool (OpenOffice do the trick easily).