Handle overlapping points becoming opaque

[keflavich]

When many points from the same data set, especially those that are physically large, are overlaid, they become opaque, e.g. TonyWong in this case:

We should allow either order or transparency to be adjusted.

[e-koch]

@keflavich @agianne -- Do we want to make this an adjustable parameter that can be set when querying? Or some clever way of adjusting alpha on a per dataset basis based on density of points?

[agianne]

@keflavich @e-koch It would be nice to have alpha adjusted on the basis of the density of points, but won't this also affect all the other points?

[e-koch]

@agianne - I was thinking we could compute the density in the plane using all of the selected data, then scale the alphas by the dataset they belong to (so right now, by author). Then each dataset has a unique alpha based on how clustered the points are.

I guess this also depends on the resolution the plot is being viewed at...