How to apply the pair syntax?

[bardo84]

Re https://discourse.julialang.org/t/algebraofgraphics-question/72982

To name axes or legends in this format you would use the pair syntax, that is x => "x" , y => "y" , grp => "group" .

How to apply the pair syntax? data(df) * mapping(:bill_length_mm => (t -> t / 10) => "bill length (cm)") appears again for the other data(df) form. (How can I understand this syntax though?) I tried some possible places. VSCode flags where the syntax is obviously wrong, other locations give run-time errors. I searched but could not find code with => in the tutorial or gallery examples.