[mxfh]

The original data would be well represented in 100x100 matrix. since the data (grades 0-100) is already discrete. Basically the first picture in the article with a alpha setting that matches 1 (1=opaque) when multiplied with the maximum number of entries per field. e.g Max entries = 5 => alpha = 1/5 = 0.2. Alternatively aggregating for 10x10 20x20 25x25 50x50 would work to if the data is too sparse. There is in need for Hex binning in this case!

Best practice: http://www.nytimes.com/interactive/2012/05/09/us/politics/sa...

[cscheid]

Alpha doesn't work additively for pretty much anydrawing packages:

http://en.wikipedia.org/wiki/Alpha_compositing

When overplotting, the usual compositing operator gives a final alpha of

1 - (1-alpha)^N

So your alpha = 1/5 overdrawn 5 times would give a final opacity of ~0.673. By its very nature, there is no alpha < 1 which when composited together a finite number of times gives alpha = 1.

[mxfh]

I was aware of that this approach towards alpha was oversimplistic to begin with, should have pointed that out. Thanks for posting the correct formula.