FIX: Gridliner should handle offset central longitudes

This merges parallel multi linestring geometries from an intersection
into a single linestring when possible. Before this, the line would start
in the middle of the axes and then go to the boundaries. The previous
logic would only take the start/end geometry points and would therefore
put the labels in the middle of the axes at "0" instead of on the edges.

lib/cartopy/mpl/gridliner.py

@@ -21,6 +21,7 @@
 import matplotlib.ticker as mticker
 import matplotlib.transforms as mtrans
 import numpy as np
+import shapely
 import shapely.geometry as sgeom
 
 import cartopy
@@ -815,17 +816,16 @@ def update_artist(artist, renderer):
                     if isinstance(intersection, sgeom.LineString):
                         intersection = [intersection]
                     elif len(intersection.geoms) > 4:
-                        # Gridline and map boundary are parallel and they
-                        # intersect themselves too much it results in a
-                        # multiline string that must be converted to a single
-                        # linestring. This is an empirical workaround for a
-                        # problem that can probably be solved in a cleaner way.
-                        xy = np.append(
-                            intersection.geoms[0].coords,
-                            intersection.geoms[-1].coords,
-                            axis=0,
-                        )
-                        intersection = [sgeom.LineString(xy)]
+                        # If lines are parallel, there will be many intersections
+                        # merge them to get only one for the calculations below
+                        merged_line = shapely.line_merge(intersection)
+                        if isinstance(merged_line, sgeom.MultiLineString):
+                            # our merge still produced a multilinestring, so
+                            # manually concatenate the original coordinates
+                            xy = np.concatenate(
+                                [inter.coords for inter in intersection.geoms], axis=0)
+                            merged_line = shapely.LineString(xy)
+                        intersection = [merged_line]
                     else:
                         intersection = intersection.geoms
                     tails = []

lib/cartopy/tests/mpl/test_gridliner.py

@@ -149,13 +149,10 @@ def test_grid_labels():
     ax.coastlines(resolution="110m")
     ax.gridlines(draw_labels=True)
 
-    # Check that adding labels to Mercator gridlines gives an error.
-    # (Currently can only label PlateCarree gridlines.)
     ax = fig.add_subplot(
         3, 2, 2, projection=ccrs.PlateCarree(central_longitude=180))
     ax.coastlines(resolution="110m")
 
-    ax.set_title('Known bug')
     gl = ax.gridlines(crs=crs_pc, draw_labels=True)
     gl.top_labels = False
     gl.left_labels = False