Update mpl_draw() to fix multigraph plots

releasenotes/notes/fix-mpl-draw-digraph-plots-aecf86738ab9b0db.yaml

@@ -0,0 +1,10 @@
+---
+fixes:
+  - |
+    Fixes the plots of multigraphs using the `mpl_draw()` function. Refer to
+    `#12345 <https://github.com/Qiskit/rustworkx/issues/774>` for more
+    details.
+other:
+  - |
+     The radius of the edges of self-loops in multigraphs is set to `0.25`. 
+     The edge labels are offset accordingly. 

rustworkx/visualization/matplotlib.py

@@ -754,6 +754,15 @@ def _connectionstyle(posA, posB, *args, **kwargs):
         else:
             line_width = width
 
+        # radius of edges
+        reverse_edge = [dst, src]
+        if (
+            len(np.where(np.all(edge_pos == reverse_edge, axis=(1, 2)))[0]) != 0
+        ):  # if reverse edge is in `edge_pos`
+            rad = 0.25
+        else:
+            rad = 0.0
+
         arrow = mpl.patches.FancyArrowPatch(
             (x1, y1),
             (x2, y2),
@@ -763,7 +772,7 @@ def _connectionstyle(posA, posB, *args, **kwargs):
             mutation_scale=mutation_scale,
             color=arrow_color,
             linewidth=line_width,
-            connectionstyle=_connectionstyle,
+            connectionstyle=connectionstyle + f", rad = {rad}",
             linestyle=style,
             zorder=1,
         )  # arrows go behind nodes
@@ -1001,6 +1010,12 @@ def draw_edge_labels(
             x1 * label_pos + x2 * (1.0 - label_pos),
             y1 * label_pos + y2 * (1.0 - label_pos),
         )
+        if (n2, n1) in labels.keys():  # loop
+            dy = np.abs(y2 - y1)
+            if n2 > n1:
+                y -= 0.25 * dy
+            else:
+                y += 0.25 * dy
 
         if rotate:
             # in degrees