multicategory sorting bugs

[alexcjohnson]

Based on a report by @nicolaskruchten

If you create multicategory data that aren't in order, there are different bugs whether the out-of-order values are within one trace or across multiple traces. See https://codepen.io/alexcjohnson/pen/vvgGOB?editors=0010

Across multiple traces (step 1): The combinations in the second trace are tacked on at the end, rather than inserted in the series after the existing values having the same outer category. Then when we're drawing labels, the outer categories are not contiguous, so the labels end up in a weird place. In this example, [4,2] is the 4th item and [4,1] is the 7th (with [5,1] and [6,2] between, from the first trace) so the 4 is drawn halfway between the 5th and 6th items... but that's on a divider between outer categories 5 and 6!

Within one trace (step 2): The combinations are correctly sorted, BUT the x data are placed in their unsorted positions.