update rank hist plot

mlair/plotting/postprocessing_plotting.py

@@ -1333,11 +1333,14 @@ class PlotRankHistogram(AbstractPlotClass):
             pdf_pages = matplotlib.backends.backend_pdf.PdfPages(plot_path)
             for ah in value[self.ahead_dim]:
                 fig, ax = plt.subplots()
-                ax.bar(value.sel({"rank_hist_type": "bins", self.ahead_dim: ah.values}),
-                       value.sel({"rank_hist_type": "freq", self.ahead_dim: ah.values}),
-                       align='center')
-                ax.set_xlabel(f"{self.target_var} (in {self.target_unit})")
-                ax.set_ylabel("freq")
+                bins = value.sel({"rank_hist_type": "bins", self.ahead_dim: ah.values})
+                freq = value.sel({"rank_hist_type": "freq", self.ahead_dim: ah.values})
+                relfreq = freq/freq.sum()
+                ax.bar(bins, relfreq, align='center', color="gray")
+                ax.hlines(1. / len(bins), 0, len(bins), color="black", linestyle="dashed")
+                ax.set_xlabel(r"Verification Rank ($n_{ens}+1$)")
+                ax.set_ylabel("Relative Frequency")
+
                 plt.title(f"{ah.values}")
                 pdf_pages.savefig()
                 plt.close('all')