[Bug] colored_tokens generates unreadable text when values are all zero

[naterush]

First - thanks for the great library! Been working well for me so far, and has a pleasant and expressive API that's been perfect for what I need.

I ran bug on Google Colab where sometimes colored_tokens function generates white text on a white background, making it unreadable. Specifically, this is when the values are all zeros - which sometimes happens, but I still want to be able to read the text!

See here:

[sjgerstner]

Hi! I have the same problem. It is not specific to Colab. It happens in a slightly more general case: Whenever all the values in the text are negative or zero, the zero values are rendered as white text on white background. As soon as at least one value is strictly positive, it's fine.
For me the problem happened in colored_tokens_multi.
Expected: black text on white background for all zero values, regardless if there are positive values around.

Update: When this happens (i.e. all values are <=0 and some are ==0), I can get around the problem by replacing the zeros with -1e-7.