Advanced tooltip customization

[sfc-gh-cheliu]

Hi there,

I want to contribute to the altair repo such that I can achieve:

1. Filter out/hide null/nan values in tooltip
2. Sort the values in tooltip

Not sure whether this is the feasible approach, but I think if alt.Chart can be built based on a dictionary, then I somehow sort and filter out some keys/values. Could you please give me some pointers where I can start?

Thanks

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