Add vertical grid lines to line graphs

This makes it possible to draw vertical grid lines. 
If `verticalGrid` is set to `true` it will draw them where the labels are. It can also be set to the same values as xLabels (`auto`, `month` and so on) to draw them at those positions. 
`verticalGridHeight` is the height of the line, by default it's the full height of the graph. It can however be changed so you only get small dashes. 
`verticalGridStartOffset` is used to extend the line under the X axis.
This fixes #254, which has lots of +1's.