I'm currently working on a project which produces a choropleth map similar to a map like this: http://dl.dropboxusercontent.com/u/54512470/Telefondaten_VIZ/index.html
That map has 2x5 classifications (5 for positive migration numbers, 5 for negative ones.)
I have different underlying numbers than the one I linked. How do I decide the following things:
- Linear or Exponential increase of the break points (10, 20, 30, 40, 50 vs. 10, 20, 40, 80, 160)? Or even square-root-shaped?
- related: If linear(ish), when is it ok to make some bins larger than others?
- 'final break point', i.e. what would be the equivalent of -500 and 500 in the linked example
Is there generic advice on these things which can be given without looking at the data?
Edit: The reason I don't want to post the data here is that I'd like to use some kind of algorithm to automatically create bins and resulting choropleth maps. Unlike the linked project, I don't have the migration data for a fixed timespan, but I have data for 2007, 2008, and so on up to 2011. I'd like a radio button to switch between the years (can combine several years).
I don't want the colors to be consitent between different different settings (i.e. + 5000 in year 2007 may have a different color than for 2010 + 2011), but to be informative for a fixed year.
One approach I was thinking of is: Take the largest absolute migration number (e.g. 1200) and round down to the nearest 50, 100, 500, 1000 and so on. This is the lower border of the dark orange, let's call this threshold number T (i.e. 1000 in this example). The threshold for the dark blue is everything below -T (i.e. -1000). The binning points of the linked project are T/10, T/5 and T/2 (in the example 100, 200, 500). I'm unsure about these particular break points. Why not T/10, T/4, T/2 (100, 250, 500)?