[Feature Request] Example featuring the NestedValue

[werner291]

What is the feature ?

The crate appears to feature the NestedValue type, which appears to respond to my need for a way to do grouped bar plots, but no example features how to use it (except maybe the small unit tests at the bottom of the file. Also, the relation between NestedValue and categorical data is not immediately clear from documentation (though I can translate from categorical to just integers)

(Optional) Why this feature is useful and how people would use the feature ?

It would help users who need hierarchical data on their axes to get started more quickly if there was an example on how to use these.