r statistical chart type docs

[cldougl]

• 2D density (similar to: https://plot.ly/python/density-plots/#2d-histogram-contour-plot-with-histogram-subplots)
• treeplot (w/ igraph) (similar to: https://plot.ly/python/tree-plots/)
• cont error bars (similar to: https://plot.ly/python/continuous-error-bars/)

[priyatharsan]

closing this issue and moving the issue tasks to #968