This issue is scoped to 2D type = date axes when there is no ticktext.

We need to add a new attribute (proposed name ticklabelmode) whose default value (instant) describes the current behaviour of our axes.

The new behaviour (period) would cause the tick label (not the tick or gridline!) to be positioned at the midpoint of the period that its current formatting represents (note: this means that it's sensitive to tickformat and tickformatstops). So for example if the ticklabel is Jan then instead of positioning it at YYYY-01-01 00:00 (the 'instant') we would position it at YYYY-01-15 12:00 (the midpoint of the 'period').