Add Line Shapes example to docs

- fig : https://plot.ly/~etpinard/436/linear-spline-vhv-hvh-vh-hv/
- inspired by : https://plot.ly/~alex/315/new-line-shape-options/
I like *Line Shapes* as it is consistent with key in question (named `'shape'` in the Line object). 

I'm thinking about calling this header *Line Shapes, Interpolation Options* ? Thoughts?