[legend] provide a label to describe each possible value represented in the legend

[markov00]

As described in #2044 we should consider a way to label the represent value in the legend.
Right now is implicit and is just a parameter shown by default with no particular meaning for a new user. The only way to understand what that value really means is by interacting a bit with the chart and discovering that feature.

We should instead provide a label/description accessible and visible to the user that describes the semantics of that value.
Similar tools offer a legend title or present the legend as a table providing that label as a column header.

Reference issues/PRs

#246

cc @gvnmagni