Add plane (N) alias for grdview

Included test for setting a plain z-axis plane at a fixed height, and a test for colouring the frontal facade too.

pygmt/base_plotting.py

@@ -239,6 +239,7 @@ def grdimage(self, grid, **kwargs):
         Jz="zscale",
         JZ="zsize",
         C="cmap",
+        N="plane",
         Q="surftype",
         p="perspective",
     )
@@ -266,6 +267,12 @@ def grdview(self, reliefgrid, **kwargs):
         cmap (C) : str
             The name of the color palette table to use.
 
+        plane (N) : float or str
+            ``level[+gfill]``.
+            Draws a plane at this z-level. If the optional color is provided via the +g
+            modifier, and the projection is not oblique, the frontal facade between the
+            plane and the data perimeter is colored.
+
         surftype (Q) : str
             Specifies cover type of the reliefgrid. Select one of following settings:
             1. 'm' for mesh plot [Default].

pygmt/tests/test_grdview.py

@@ -122,3 +122,27 @@ def test_grdview_with_cmap_for_perspective_surface_plot(grid):
         zscale=0.005,
     )
     return fig
+
+
+@pytest.mark.mpl_image_compare
+def test_grdview_on_a_plane(grid):
+    """
+    Run grdview by passing in a reliefgrid and plotting it on a z-plane, while settings
+    a 3D perspective viewpoint.
+    """
+    fig = Figure()
+    fig.grdview(reliefgrid=grid, plane=-4000, perspective=[225, 30], zscale=0.005)
+    return fig
+
+
+@pytest.mark.mpl_image_compare
+def test_grdview_on_a_plane_with_colored_frontal_facade(grid):
+    """
+    Run grdview by passing in a reliefgrid and plotting it on a z-plane whose frontal
+    facade is colored gray, while setting a 3D perspective viewpoint.
+    """
+    fig = Figure()
+    fig.grdview(
+        reliefgrid=grid, plane="-4000+ggray", perspective=[225, 30], zscale=0.005
+    )
+    return fig