Legend of multi-box plot and CSI plot do not increase in txt size

[rschwant]

I found out when making my AGU poster, that the legend of multi-box plot and CSI plot do not increase when txt size is increased in the yaml file. Let's connect this to connect with the "text_kwargs: fontsize:" yaml option. This is non-urgent and can be addressed after AGU and holiday's. I'm just noting it here so we do not forget to fix it.