start filled area from y 0 instead of bottom

This fixes that when drawing filled area graphs with negative values the area is filled from the bottom of the graph, not from the 0 point on the y axis.

This pull reqeust is a merge/cleanup of https://github.com/graphite-project/graphite-web/pull/249 that one can be closed.

 When applying to head please check https://github.com/graphite-project/graphite-web/pull/307 which fixes a regression on areaMode=all.