Revert "Temporarily remove the gallery example for grdgradient"

This reverts commit ad6c7dc.

Author: seisman

examples/gallery/images/grdgradient.py

@@ -0,0 +1,54 @@
+"""
+Calculating grid gradient and radiance
+--------------------------------------
+The :meth:`pygmt.grdgradient` method calculates the gradient of a grid file.
+In the example shown below we will see how to calculate a hillshade map based
+on a Data Elevation Model (DEM). As input :meth:`pygmt.grdgradient` gets
+a :class:`xarray.DataArray` object or a path string to a grid file, calculates
+the respective gradient and returns it as an :class:`xarray.DataArray` object.
+We will use the ``radiance`` parameter in order to set the illumination source
+direction and altitude.
+"""
+
+import pygmt
+
+# Define region of interest around Yosemite valley
+region = [-119.825, -119.4, 37.6, 37.825]
+
+# Load sample grid (3 arc second global relief) in target area
+grid = pygmt.datasets.load_earth_relief(resolution="03s", region=region)
+
+# calculate the reflection of a light source projecting from west to east
+# (azimuth of 270 degrees) and at a latitude of 30 degrees from the horizon
+dgrid = pygmt.grdgradient(grid=grid, radiance=[270, 30])
+
+fig = pygmt.Figure()
+# define figure configuration
+pygmt.config(FORMAT_GEO_MAP="ddd.x", MAP_FRAME_TYPE="plain")
+
+# --------------- plotting the original Data Elevation Model -----------
+
+pygmt.makecpt(cmap="gray", series=[200, 4000, 10])
+fig.grdimage(
+    grid=grid,
+    projection="M12c",
+    frame=['WSrt+t"Original Data Elevation Model"', "xa0.1", "ya0.1"],
+    cmap=True,
+)
+
+fig.colorbar(position="JML+o1.4c/0c+w7c/0.5c", frame=["xa1000f500+lElevation", "y+lm"])
+
+# --------------- plotting the hillshade map -----------
+
+# Shift plot origin of the second map by 12.5 cm in x direction
+fig.shift_origin(xshift="12.5c")
+
+pygmt.makecpt(cmap="gray", series=[-1.5, 0.3, 0.01])
+fig.grdimage(
+    grid=dgrid,
+    projection="M12c",
+    frame=['lSEt+t"Hillshade Map"', "xa0.1", "ya0.1"],
+    cmap=True,
+)
+
+fig.show()