series.bars.align : "center" fix

The align option for a bar sets whatever is chosen on the grid line (i.e. the left-most portion of the bar starts at zero when left is selected), so when center was chosen the math was incorrect. This fixes it.

Screenshot of problem:
![image](https://cloud.githubusercontent.com/assets/1518719/3818773/12bc4514-1ce4-11e4-9040-df0e8e1114e9.png)

Screenshot of fix:
![image](https://cloud.githubusercontent.com/assets/1518719/3818800/4d9f8556-1ce4-11e4-8eec-cdf84ca2df53.png)