Add gallery example for grdview (#502)

* Add gallery example for grdview

* Add second line before function definition

* Fix incorrect method syntax

* Use "meth" instead of "func"

* import xarray as xr

Co-authored-by: Dongdong Tian <seisman.info@gmail.com>

* Finish updating to use xr

Co-authored-by: Dongdong Tian <seisman.info@gmail.com>

examples/gallery/grid/grdview_surface.py

@@ -0,0 +1,50 @@
+"""
+Plotting a surface
+------------------
+
+The :meth:`pygmt.Figure.grdview()` method can plot 3-D surfaces with ``surftype="s"``. Here,
+we supply the data as an :class:`xarray.DataArray` with the coordinate vectors ``x`` and
+``y`` defined. Note that the ``perspective`` argument here controls the azimuth and
+elevation angle of the view. We provide a list of two arguments to ``frame`` — the
+second argument, prepended with ``"z"``, specifies the :math:`z`-axis frame attributes.
+Specifying the same scale for the ``projection`` and ``zcale`` arguments ensures equal
+axis scaling.
+"""
+
+import pygmt
+import numpy as np
+import xarray as xr
+
+
+# Define an interesting function of two variables, see:
+# https://en.wikipedia.org/wiki/Ackley_function
+def ackley(x, y):
+    return (
+        -20 * np.exp(-0.2 * np.sqrt(0.5 * (x ** 2 + y ** 2)))
+        - np.exp(0.5 * (np.cos(2 * np.pi * x) + np.cos(2 * np.pi * y)))
+        + np.exp(1)
+        + 20
+    )
+
+
+# Create gridded data
+INC = 0.05
+x = np.arange(-5, 5 + INC, INC)
+y = np.arange(-5, 5 + INC, INC)
+data = xr.DataArray(ackley(*np.meshgrid(x, y)), coords=(x, y))
+
+fig = pygmt.Figure()
+
+# Plot grid as a 3-D surface
+SCALE = 0.2  # [inches]
+fig.grdview(
+    data,
+    frame=["a5f1", "za5f1"],
+    projection=f"x{SCALE}i",
+    zscale=f"{SCALE}i",
+    surftype="s",
+    cmap="roma",
+    perspective="135/30",
+)
+
+fig.show()