Display cursor coordinates for all axes twinned with the current one.

Author: anntzer

lib/matplotlib/axes/_base.py

@@ -3976,10 +3976,20 @@ def format_ydata(self, y):
 
     def format_coord(self, x, y):
         """Return a format string formatting the *x*, *y* coordinates."""
-        return "x={} y={}".format(
-            "???" if x is None else self.format_xdata(x),
-            "???" if y is None else self.format_ydata(y),
-        )
+        twins = self._twinned_axes.get_siblings(self)
+        if len(twins) == 1:
+            return "(x, y) = ({}, {})".format(
+                "???" if x is None else self.format_xdata(x),
+                "???" if y is None else self.format_ydata(y))
+        screen_xy = self.transData.transform((x, y))
+        xy_strs = []
+        # Retrieve twins in the order of self.figure.axes to sort tied zorders (which is
+        # the common case) by the order in which they are added to the figure.
+        for ax in sorted(twins, key=attrgetter("zorder")):
+            data_x, data_y = ax.transData.inverted().transform(screen_xy)
+            xy_strs.append(
+                "({}, {})".format(ax.format_xdata(data_x), ax.format_ydata(data_y)))
+        return "(x, y) = {}".format(" | ".join(xy_strs))
 
     def minorticks_on(self):
         """

lib/matplotlib/tests/test_backend_bases.py

@@ -1,5 +1,3 @@
-import re
-
 from matplotlib import path, transforms
 from matplotlib.backend_bases import (
     FigureCanvasBase, KeyEvent, LocationEvent, MouseButton, MouseEvent,
@@ -123,11 +121,21 @@ def test_location_event_position(x, y):
         assert event.y == int(y)
         assert isinstance(event.y, int)
     if x is not None and y is not None:
-        assert re.match(
-            f"x={ax.format_xdata(x)} +y={ax.format_ydata(y)}",
-            ax.format_coord(x, y))
+        assert (ax.format_coord(x, y)
+                == f"(x, y) = ({ax.format_xdata(x)}, {ax.format_ydata(y)})")
         ax.fmt_xdata = ax.fmt_ydata = lambda x: "foo"
-        assert re.match("x=foo +y=foo", ax.format_coord(x, y))
+        assert ax.format_coord(x, y) == "(x, y) = (foo, foo)"
+
+
+def test_location_event_position_twin():
+    fig, ax = plt.subplots()
+    ax.set(xlim=(0, 10), ylim=(0, 20))
+    assert ax.format_coord(5., 5.) == "(x, y) = (5.00, 5.00)"
+    ax.twinx().set(ylim=(0, 40))
+    assert ax.format_coord(5., 5.) == "(x, y) = (5.00, 5.00) | (5.00, 10.0)"
+    ax.twiny().set(xlim=(0, 5))
+    assert (ax.format_coord(5., 5.)
+            == "(x, y) = (5.00, 5.00) | (5.00, 10.0) | (2.50, 5.00)")
 
 
 def test_pick():