feat: add function to merge handles for identical labels in legend (#516)

* Add function to merge handles for identical labels in legend

This function is useful, when using multiple plotting functions and one only wants to use a single label.
This function combines all the handles for identical labels into a single handle.

* Make merge_legend_handles_labels function available

Author: cverstege

src/mplhep/__init__.py

@@ -18,6 +18,7 @@
     hist2dplot,
     histplot,
     make_square_add_cbar,
+    merge_legend_handles_labels,
     mpl_magic,
     rescale_to_axessize,
     sort_legend,
@@ -70,6 +71,7 @@
     "rescale_to_axessize",
     "box_aspect",
     "make_square_add_cbar",
+    "merge_legend_handles_labels",
     "append_axes",
     "sort_legend",
     "save_variations",

src/mplhep/plot.py

@@ -1408,3 +1408,28 @@ def sort_legend(ax, order=None):
     if isinstance(order, OrderedDict):
         ordered_label_list = [order[k] for k in ordered_label_list]
     return ordered_label_values, ordered_label_list
+
+
+def merge_legend_handles_labels(handles, labels):
+    """
+    Merge handles for identical labels.
+    This is useful when combining multiple plot functions into a single label.
+
+    handles : List of handles
+    labels : List of labels
+    """
+
+    seen_labels = []
+    seen_label_handles = []
+    for handle, label in zip(handles, labels):
+        if label not in seen_labels:
+            seen_labels.append(label)
+            seen_label_handles.append([handle])
+        else:
+            idx = seen_labels.index(label)
+            seen_label_handles[idx].append(handle)
+
+    for i in range(len(seen_labels)):
+        seen_label_handles[i] = tuple(seen_label_handles[i])
+
+    return seen_label_handles, seen_labels