pandas sort issue

in ggplot/ggplot.py line 602 is calling
` fill_levels = self.data[[fillcol_raw, fillcol]].sort(fillcol_raw)[fillcol].unique()`
instead of
` fill_levels = self.data[[fillcol_raw, fillcol]].sort_values(by=fillcol_raw)[fillcol].unique()`

as with new pandas update 'sort_value' / deprecating 'sort'