aduan5567-netizen · Sep 19, 2026 · Sep 20, 2026 · Sep 20, 2026
diff --git a/control/grid.py b/control/grid.py
@@ -31,53 +31,6 @@ def __call__(self, direction, factor, values):
         return ret
 
 
-class ModifiedExtremeFinderCycle(angle_helper.ExtremeFinderCycle):
-    """Changed to allow only left hand-side polar grid.
-
-    https://matplotlib.org/_modules/mpl_toolkits/axisartist/angle_helper.html#ExtremeFinderCycle.__call__
-    """
-    def __call__(self, transform_xy, x1, y1, x2, y2):
-        x, y = np.meshgrid(
-            np.linspace(x1, x2, self.nx), np.linspace(y1, y2, self.ny))
-        lon, lat = transform_xy(np.ravel(x), np.ravel(y))
-
-        with np.errstate(invalid='ignore'):
-            if self.lon_cycle is not None:
-                lon0 = np.nanmin(lon)
-                # Changed from 180 to 360 to be able to span only
-                # 90-270 (left hand side)
-                lon -= 360. * ((lon - lon0) > 360.)
-            if self.lat_cycle is not None:  # pragma: no cover
-                lat0 = np.nanmin(lat)
-                lat -= 360. * ((lat - lat0) > 180.)
-
-        lon_min, lon_max = np.nanmin(lon), np.nanmax(lon)
-        lat_min, lat_max = np.nanmin(lat), np.nanmax(lat)
-
-        lon_min, lon_max, lat_min, lat_max = \
-            self._add_pad(lon_min, lon_max, lat_min, lat_max)
-
-        # check cycle
-        if self.lon_cycle:
-            lon_max = min(lon_max, lon_min + self.lon_cycle)
-        if self.lat_cycle:  # pragma: no cover
-            lat_max = min(lat_max, lat_min + self.lat_cycle)
-
-        if self.lon_minmax is not None:
-            min0 = self.lon_minmax[0]
-            lon_min = max(min0, lon_min)
-            max0 = self.lon_minmax[1]
-            lon_max = min(max0, lon_max)
-
-        if self.lat_minmax is not None:
-            min0 = self.lat_minmax[0]
-            lat_min = max(min0, lat_min)
-            max0 = self.lat_minmax[1]
-            lat_max = min(max0, lat_max)
-
-        return lon_min, lon_max, lat_min, lat_max
-
-
 def sgrid(subplot=(1, 1, 1), scaling=None):
     # From matplotlib demos:
     # https://matplotlib.org/gallery/axisartist/demo_curvelinear_grid.html
@@ -87,14 +40,14 @@ def sgrid(subplot=(1, 1, 1), scaling=None):
     # system in degrees
     tr = Affine2D().scale(np.pi/180., 1.) + PolarAxes.PolarTransform()
 
-    # polar projection, which involves cycle, and also has limits in
-    # its coordinates, needs a special method to find the extremes
-    # (min, max of the coordinate within the view).
+    # The s-plane grid spans the fixed longitude interval 90-270 degrees.
+    # Disable cycle unwrapping at the 180-degree boundary and use lon_minmax
+    # to clip the grid to the left half-plane.
 
     # 20, 20 : number of sampling points along x, y direction
     sampling_points = 20
-    extreme_finder = ModifiedExtremeFinderCycle(
-        sampling_points, sampling_points, lon_cycle=360, lat_cycle=None,
+    extreme_finder = angle_helper.ExtremeFinderCycle(
+        sampling_points, sampling_points, lon_cycle=None, lat_cycle=None,
         lon_minmax=(90, 270), lat_minmax=(0, np.inf),)
 
     grid_locator1 = angle_helper.LocatorDMS(15)

diff --git a/control/tests/grid_test.py b/control/tests/grid_test.py
@@ -0,0 +1,14 @@
+import numpy as np
+import matplotlib.pyplot as plt
+
+from control.grid import sgrid
+
+
+def test_sgrid_does_not_unwrap_longitude(mplcleanup):
+    ax, fig = sgrid()
+    finder = ax.get_grid_helper().grid_finder.extreme_finder
+
+    assert finder.lon_cycle is None
+    limits = finder(lambda x, y: (x, y), 90, 0, 270.001, 1)
+    np.testing.assert_allclose(limits, (90, 270, 0, 1.05))
+    plt.close(fig)