Enable Grapher to have horizontal axis labels

[CGiattino]

Core problem

Currently on Grapher we have vertical text on the vertical axis label, which is 1) difficult to read and 2) causes us to frequently define that axis in the subtitle, which is redundant. See this Slack message for more info and examples, plus a larger discussion.

Proposed solution

Enable Grapher to have horizontal axis labels for the vertical axis. It would also be nice to be able to choose where both the vertical and horizontal axis labels are — often outside the chart area is easiest to do and works fine, but having an option to move them inside the chart area would also be great.

Alternatives

None spring to mind.

Context

Again, this Slack thread has lots of context.

[danyx23]

We want to do this but the details are tricky and it needs some more design. Marwa said she would need to see some "bad cases" to play a bit with designs. We said that we would have to visually tie the axis and axis labels together so that it doesn't look like another line of subtitle.

We discussed moving it just above the chart area but it would be even nicer of course if we could use empty chart area in scatter plots for showing these axis labels.