Add line_style attribute to graph

- line_style can be :stepped, :interpolated or nil
- Add documentation around the fact that line_style
  is required when making stacked graph data points.
Merge pull request #21 from wanelo-chef/stacked_sets

Add line_style attribute to graph
Graph line style defaults to interpolated
Merge pull request #22 from wanelo-chef/line_style_default

Graph line style defaults to interpolated