_updateProjector taken from NewStylePlot

As taken from #1010 

"Plot.Pie extends Plot, not NewStylePlot, but implements the addDataset() call. As a consequence, pie.dataset() always returns a dummy dataset with no data. When pie goes to compute extent, it tries to apply the accessor over the dummy dataset (as per the implementation of _updateProjector; see plot.ts line 208), and so of course gets nothing. Consequently, the domain for the Scale.Color is not updated properly and the Legend never draws." - @jtlan 

NewStylePlots handle this case so I'm just using their logic.  This is essentially a pain point from NewStylePlots not being the main class for plots.