Color-coding + alpha for density

[samverstocken]

Hi all,

So far, this package has been really useful for me.
If anyone is interested, I was able to do some hacks to simultaneously display auxilary information (a third axis) with a colormap while using the density of scatter points to render the transparency.

I made a series of test plots to show what I mean:

Just the density:

Just the third axis as colors:

Now with BOTH density as transparency and third axis as colors (the density is converted into alpha values on a linear scale)

Now the same, but with the alpha rendered on a logarithmic scale (i've implemented this as a flag for the scatter_density function):

I added another option to enhance the lower transparencies, still on log scale:

If someone is interested in this functionality (I couldn't find any other package that does this..), let me know!