[AbstractPlot] - Updating scale extent obeys dataset key ordering

Extent calculation in an AbstractPlot used to loop through all the keys in the key2PlotDatasetKey map.  This does not necessarily obey the order mandated by datasetOrder() so extent calculations that are order-based can be incorrect.

Extent calculation now uses datasetKeysInOrder to keep the extent calculation in accordance with the dataset order

QE: The bug that currently exists is that the scale domains are calculated in the order that seems to be in the order they are added to the plot rather than the determined datasetOrder.  The scale domains should be calculated as you expect from the datasetOrder.

Fixes #1406