Add usage of z value in ternary dataset

pygmt/datasets/samples.py

@@ -369,16 +369,16 @@ def _load_rock_sample_compositions():
     Returns
     -------
     data : pandas.DataFrame
-        The data table with columns "water", "air", and "limestone".
+        The data table with columns "limestone", "water", "air",
+        and "permittivity".
     """
 
     fname = which("@ternary.txt", download="c")
     return pd.read_csv(
         fname,
         delim_whitespace=True,
         header=None,
-        names=["water", "air", "limestone"],
-        usecols=(0, 1, 2),
+        names=["limestone", "water", "air", "permittivity"],
     )
 
 

pygmt/tests/test_datasets_samples.py

@@ -185,10 +185,12 @@ def test_rock_sample_compositions():
     Check that the @ternary.txt dataset loads without errors.
     """
     data = load_sample_data(name="rock_compositions")
-    assert data.shape == (1000, 3)
+    assert data.shape == (1000, 4)
+    assert data["limestone"].min() == 0
+    assert data["limestone"].max() == 1
     assert data["water"].min() == 0
-    assert data["water"].max() == 1
+    assert data["water"].max() == 0.921
     assert data["air"].min() == 0
-    assert data["air"].max() == 0.921
-    assert data["limestone"].min() == 0
-    assert data["limestone"].max() == 0.981
+    assert data["air"].max() == 0.981
+    assert data["permittivity"].min() == 1.041
+    assert data["permittivity"].max() == 70.844