Summary:
Until now, users could not pick the column that's currently serving as
the x axis.
To do that, chart needs to support having the x axis undefined.
This diff do just that:
- allow x axis to be undefined
- allow setting x axis from group data
Given that charts axis are stored as indexes of the section view
fields array, implementation required introduction of an extra chart
options: `isAxisUndefined`.
Test Plan: Updates existing test and adds new one.
Reviewers: jarek
Reviewed By: jarek
Subscribers: jarek
Differential Revision: https://phab.getgrist.com/D3304
76481d21e0