[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

Fixes #1406