Fix problems with graphics

[khider]

This fixes two issues encountered with EnsembleSeries and EnsembleGeoSeries graphics:

1. Issue plot_trace legend now shows the same name for all traces instead of just doing one #602 Changes to the Matplotlib API required to put a label with an underscore on the traces to avoid having a legend entry for everything since these are ignored. Another solution was found: passing the handles directly to legend; however this would result in the traces showing up again is another line was added to the plot.
2. Issue Dashboard not respecting default plot colors #598 The code was lifted directly from the GeoSeries.dashboard resulting in self.archiveType being always None. Needed to set it to the archiveType in the first entry in the series_list.