AxLines do not plot on histograms

[Hydrowelder]

When adding AxLines to histogram entries, the lines do not appear

[Hydrowelder]

This could be fixed by returning saf (SingleAxisFigure) from XYDataPlotter.handle_points_and_traces and passing it into Histogrammer.handle_histogram_entries rather than making a new SingleAxisFigure in the histogrammer. Then the SingleAxisFigure should only be deleted at the end of DataProductCollection.process_collection