Feature range should be based on feature count

[crschmidt@…]

It would be nice if the automatic feature-range styling support was based on the number of features in a given class, rather than raw values.

For population of world countries, dividing range of 0 -> 1.2 billion into 5 means that everything but china falls into one category, making a rather uninteresting map.

If instead, 1/5th of the countries were in each category, the map would be more useful with the automatic ranging.

[mhugent]

In svn head (0.9), there is now a quantile function for graduated classification