add_vline and the like work on single plots made without make_subplots

Author: nicholas-esterer

packages/python/plotly/plotly/basedatatypes.py

@@ -3507,6 +3507,9 @@ def _make_paper_spanning_shape(self, direction, shape):
         axis_num = shape[ref].strip(axis)
         axis_layout_key = axis_layout_key_template % (axis_num,)
         domain = self.layout[axis_layout_key]["domain"]
+        if domain is None:
+            # This could occur if the plot was not made with make_subplots
+            domain = [0, 1]
         try:
             shape[axis + "0"], shape[axis + "1"] = domain
         except KeyError as e: