Fix type in dot graph export

Author: AlexanderFabisch

pytransform3d/transform_manager/_transform_manager.py

@@ -260,22 +260,22 @@ def write_png(self, filename, prog=None):  # pragma: no cover
 
         for frame in self.nodes:
             node = pydot.Node(
-                _dot_display_name(frame), style="filled",
+                _dot_display_name(str(frame)), style="filled",
                 fillcolor=frame_color, shape="egg")
             graph.add_node(node)
         for frames, A2B in self._transforms.items():
             frame_a, frame_b = frames
             connection_name = "%s to %s\n%s" % (
-                _dot_display_name(frame_a), _dot_display_name(frame_b),
-                str(np.round(A2B, 3)))
+                _dot_display_name(str(frame_a)),
+                _dot_display_name(str(frame_b)), str(np.round(A2B, 3)))
             node = pydot.Node(
                 connection_name, style="filled", fillcolor=connection_color,
                 shape="note")
             graph.add_node(node)
-            a_name = _dot_display_name(frame_a)
+            a_name = _dot_display_name(str(frame_a))
             a_edge = pydot.Edge(connection_name, a_name, penwidth=3)
             graph.add_edge(a_edge)
-            b_name = _dot_display_name(frame_b)
+            b_name = _dot_display_name(str(frame_b))
             b_edge = pydot.Edge(connection_name, b_name, penwidth=3)
             graph.add_edge(b_edge)