Add regression test for grdimage plotting an xarray.DataArray grid subset (#1314)

Ensure that a sliced xarray.DataArray grid is plotted correctly
on a global Mollweide projection map. This is a regression test
to ensure that the bug reported in #732 does not happen in the future.
Cross-reference upstream issue
GenericMappingTools/gmt#5180, and PR
GenericMappingTools/gmt#5181 where the
bug was fixed.

Co-authored-by: Dongdong Tian <[email protected]>

Author: weiji14

pygmt/tests/baseline/test_grdimage_global_subset.png.dvc

@@ -0,0 +1,4 @@
+outs:
+- md5: 8f38333a67076f0125cb2d1828aa244e
+  size: 39288
+  path: test_grdimage_global_subset.png

pygmt/tests/test_grdimage.py

@@ -18,6 +18,18 @@ def fixture_grid():
     return load_earth_relief(registration="gridline")
 
 
+@pytest.fixture(scope="module", name="grid_360")
+def fixture_grid_360(grid):
+    """
+    Earth relief grid with longitude range from 0 to 360 (instead of -180 to
+    180).
+    """
+    _grid = grid.copy()  # get a copy of original earth_relief grid
+    _grid.encoding.pop("source")  # unlink earth_relief NetCDF source
+    _grid["lon"] = np.arange(0, 361, 1)  # convert longitude from -180:180 to 0:360
+    return _grid
+
+
 @pytest.fixture(scope="module", name="xrgrid")
 def fixture_xrgrid():
     """
@@ -131,6 +143,29 @@ def test_grdimage_over_dateline(xrgrid):
     return fig
 
 
+@pytest.mark.mpl_image_compare
+def test_grdimage_global_subset(grid_360):
+    """
+    Ensure subsets of grids are plotted correctly on a global map.
+
+    Specifically checking that xarray.DataArray grids can wrap around the left
+    and right sides on a Mollweide projection (W) plot correctly. Note that a
+    Cartesian grid is used here instead of a Geographic grid (i.e.
+    GMT_GRID_IS_CARTESIAN). This is a regression test for
+    https://github.com/GenericMappingTools/pygmt/issues/732.
+    """
+    # Get a slice of South America and Africa only (lat=-90:31, lon=-180:41)
+    sliced_grid = grid_360[0:121, 0:221]
+    assert sliced_grid.gmt.registration == 0  # gridline registration
+    assert sliced_grid.gmt.gtype == 0  # Cartesian coordinate system
+
+    fig = Figure()
+    fig.grdimage(
+        grid=sliced_grid, cmap="vik", region="g", projection="W0/3.5c", frame=True
+    )
+    return fig
+
+
 @check_figures_equal()
 @pytest.mark.parametrize("lon0", [0, 123, 180])
 @pytest.mark.parametrize("proj_type", ["H", "W"])