Line graph for better display of weight and similarily trending data

[GoogleCodeExporter]

Weight rarely changes more than ~5% over the course of a single month. So it 
doesn't display very well on any of the existing graphs. I believe a line graph 
is needed where only the delta is shown for the given timespan. This way the 
general trends can be spotted more easily. Especially when I've chosen a larger 
timespan such as 6 months.

Original issue reported on code.google.com by hardi.kovamees on 9 Jan 2011 at 4:51