Add gallery example for Figure.ternary method

examples/gallery/basemaps/ternary.py

@@ -0,0 +1,44 @@
+"""
+Ternary diagram
+---------------
+The :meth:`pygmt.Figure.ternary` method can draw ternary diagrams. The example
+shows how to plot circles with a diameter of 0.1 centimeters
+(``style="c0.1c"``) on a 10-centimeter-wide (``width="10c"``) ternary diagram
+at the positions listed in the first three columns of the sample dataset
+``rock_compositions``, with default annotations and gridline spacings, using
+the specified labeling defined via ``alabel``, ``blabel``, and ``clabel``.
+Points are colored based on the values given in the fourth columns of the
+sample dataset via ``cmap=True``.
+"""
+
+import pygmt
+
+fig = pygmt.Figure()
+
+# Load sample data
+data = pygmt.datasets.load_sample_data(name="rock_compositions")
+
+# Define a colormap to be used for the values given in the fourth column
+# of the input dataset
+pygmt.makecpt(cmap="batlow", series=[0, 80, 10])
+
+fig.ternary(
+    data,
+    region=[0, 100, 0, 100, 0, 100],
+    width="10c",
+    style="c0.1c",
+    alabel="Limestone",
+    blabel="Water",
+    clabel="Air",
+    cmap=True,
+    frame=[
+        "aafg+lLimestone component+u %",
+        "bafg+lWater component+u %",
+        "cagf+lAir component+u %",
+    ],
+)
+
+# Add a colorbar indicating the values given in the fourth column of
+# the input dataset
+fig.colorbar(position="JBC+o0c/1.5c", frame=["x+lPermittivity"])
+fig.show()