I've just started to play with altair, using the diamonds dataset. Here is the notebook to clarify what I did https://gist.github.com/pierre-haessig/09fa9268aa0a0e7d91356f681f96ca18

Since, I'm not familiar with altair, I maybe missed something, but I've got the feeling that ordered Categorical types from pandas are not supported.

Indeed, if I use a color='cut' encoding, when cut is a pandas Series with an ordered category dtype, I get by default a nominal type of coloring (with "unordered" colors).

On the other hand, if I force the use of ordered with color='cut:O', I indeed get the ordered colored (the shades of green), but the order is wrong! (I get Fair, Good, Ideal, Premium, Very Good, while the correct order is 'Fair', 'Good', 'Very Good', 'Premium', 'Ideal', as manually defined in pandas' category)