Compare the results of :
http://chart.apis.google.com/chart?cht= ... xr=0,-5,10
to
http://localhost:8080/chart?cht=bvg&chs ... xr=0,-5,10
I think that there is a problem in the data scaling algorithm because you substract min to the value (and so if min is negative, that would be a problem).
Correcting the data scaling does not seem to work (after, there are problem on axis range).
I'll be glad to provide more info if needed