added docstring and exemplary graph

mlair/plotting/data_insight_plotting.py

@@ -445,6 +445,16 @@ class PlotAvailabilityHistogram(AbstractPlotClass):  # pragma: no cover
 
 
 class PlotDataHistogram(AbstractPlotClass):  # pragma: no cover
+    """
+    Plot histogram on transformed input and target data. This data is the same that the model sees during training. No
+    plots are create for the original values space (raw / unformatted data). This plot method will create a histogram
+    for input and target each comparing the subsets train, val and test, as well as a distinct one for the three
+    subsets.
+
+    .. image:: ../../../../../_source/_plots/datahistogram.png
+        :width: 400
+
+    """
 
     def __init__(self, generators: Dict[str, DataCollection], plot_folder: str = ".", plot_name="histogram",
                  variables_dim="variables", time_dim="datetime", window_dim="window"):