small number of distinct, sequential numbers should produce category with sequential color scheme

[andy-esch]

sometimes a number column has a small number of distinct values and is sequential. This should produce a category visualization but still have sequential colors.

For instance, this dataset has numbers 0, 1, 2, 3, 4 to signify the drought severity. Pecan currently suggests a choropleth vis and gives seven categories in the legend.

cc @makella