Add Density-Scatter Plot

[ZimmerD]

Description

The current example for Histogram2D features an example of a combined Chart using a Histogram2DContour and Point Chart:

let histogramContour =
    [
        Chart.Histogram2DContour (x,y,Line=Line.init(Width=0.))
        Chart.Point(x,y,Opacity=0.3)
    ]
    |> Chart.combine

However, the resulting result has traces of different sizes, resulting in a slight gray, line on top of the chart (see docs). You can style the Chart to get rid of it, however I think it would be worthwhile to add a Chart type, which wraps the functionality to be able to write:

let histogramContour = Chart.DensityScatter (x,y) // naming not final. just suggestion    

and get a nicely styled version of the above.

[kMutagene]

@ZimmerD what do you think of the name Chart.PointDensity ?

[ZimmerD]

Sounds good to me!