Add a sample dataset for ternary examples (GenericMappingTools#2211)

Co-authored-by: Will Schlitzer <schlitzer90@gmail.com>
Co-authored-by: Dongdong Tian <seisman.info@gmail.com>

pygmt/datasets/samples.py

@@ -32,8 +32,9 @@ def list_sample_data():
         "mars_shape": "Table of topographic signature of the hemispheric dichotomy of "
         " Mars from Smith and Zuber (1996)",
         "maunaloa_co2": "Table of CO2 readings from Mauna Loa",
-        "ocean_ridge_points": "Table of ocean ridge points for the entire world",
         "notre_dame_topography": "Table 5.11 in Davis: Statistics and Data Analysis in Geology",
+        "ocean_ridge_points": "Table of ocean ridge points for the entire world",
+        "rock_compositions": "Table of rock sample compositions",
         "usgs_quakes": "Table of global earthquakes from the USGS",
     }
     return names
@@ -80,6 +81,7 @@ def load_sample_data(name):
 
     # Dictionary of private load functions
     load_func = {
+        "rock_compositions": _load_rock_sample_compositions,
         "earth_relief_holes": _load_earth_relief_holes,
         "maunaloa_co2": _load_maunaloa_co2,
         "notre_dame_topography": _load_notre_dame_topography,
@@ -360,6 +362,26 @@ def load_mars_shape(**kwargs):
     return data
 
 
+def _load_rock_sample_compositions():
+    """
+    Loads a table of rock sample compositions.
+
+    Returns
+    -------
+    data : pandas.DataFrame
+        The data table with columns "water", "air", and "limestone".
+    """
+
+    fname = which("@ternary.txt", download="c")
+    return pd.read_csv(
+        fname,
+        delim_whitespace=True,
+        header=None,
+        names=["water", "air", "limestone"],
+        usecols=(0, 1, 2),
+    )
+
+
 def _load_notre_dame_topography():
     """
     Load Table 5.11 in Davis: Statistics and Data Analysis in Geology.

pygmt/tests/test_datasets_samples.py

@@ -186,3 +186,18 @@ def test_maunaloa_co2():
     assert summary.loc["max", "date"] == 2019.3699
     assert summary.loc["min", "co2_ppm"] == 313.2
     assert summary.loc["max", "co2_ppm"] == 414.83
+
+
+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)
+    summary = data.describe()
+    assert summary.loc["min", "water"] == 0
+    assert summary.loc["max", "water"] == 1
+    assert summary.loc["min", "air"] == 0
+    assert summary.loc["max", "air"] == 0.921
+    assert summary.loc["min", "limestone"] == 0
+    assert summary.loc["max", "limestone"] == 0.981