Figure.ternary: Add parameters alabel, blabel, and clabel

pygmt/src/ternary.py

@@ -23,7 +23,7 @@
     t="transparency",
 )
 @kwargs_to_strings(R="sequence", c="sequence_comma", p="sequence")
-def ternary(self, data, **kwargs):
+def ternary(self, data, alabel=None, blabel=None, clabel=None, **kwargs):
     r"""
     Reads (*a*,\ *b*,\ *c*\ [,\ *z*]) records from *data* and plots symbols at
     those locations on a ternary diagram. If a symbol is selected and no symbol
@@ -54,6 +54,14 @@ def ternary(self, data, **kwargs):
         and **c**.
     {cmap}
     {fill}
+    alabel : str
+        Set the label for the *a* vertex where the component is 100%. The
+        label is placed at a distance of three times the
+        :gmt-term:`MAP_LABEL_OFFSET` setting from the corner.
+    blabel : str
+        Set the label for the *b* vertex where the component is 100%.
+    clabel : str
+        Set the label for the *c* vertex where the component is 100%.
     style : str
         *symbol*\[\ *size*].
         Plot individual symbols in a ternary diagram.
@@ -66,6 +74,12 @@ def ternary(self, data, **kwargs):
     {transparency}
     """
     kwargs = self._preprocess(**kwargs)  # pylint: disable=protected-access
+
+    alabel = str(alabel) if alabel is not None else "-"
+    blabel = str(blabel) if blabel is not None else "-"
+    clabel = str(clabel) if clabel is not None else "-"
+    kwargs["L"] = "/".join([alabel, blabel, clabel])
+
     with Session() as lib:
         file_context = lib.virtualfile_from_data(check_kind="vector", data=data)
         with file_context as infile:

pygmt/tests/baseline/test_ternary_1_label.png.dvc

@@ -0,0 +1,4 @@
+outs:
+- md5: 7278b6cdaf0076840927380a22d03846
+  size: 63228
+  path: test_ternary_1_label.png

pygmt/tests/baseline/test_ternary_3_labels.png.dvc

@@ -0,0 +1,4 @@
+outs:
+- md5: e303384766b3b5e31cad90a9b5e61136
+  size: 64570
+  path: test_ternary_3_labels.png

pygmt/tests/test_ternary.py

@@ -62,3 +62,43 @@ def test_ternary(array):
         pen="thinnest",
     )
     return fig
+
+
+@pytest.mark.mpl_image_compare
+def test_ternary_3_labels(array):
+    """
+    Test plotting a ternary chart with 3 labels.
+    """
+    fig = Figure()
+    fig.ternary(
+        data=array,
+        region=[0, 100, 0, 100, 0, 100],
+        cmap="red,orange,yellow,green,blue,violet",
+        width="10c",
+        alabel="A",
+        blabel="B",
+        clabel="C",
+        frame=["bafg+lAir", "cafg+lLimestone", "aafg+lWater"],
+        style="c0.1c",
+        pen="thinnest",
+    )
+    return fig
+
+
+@pytest.mark.mpl_image_compare
+def test_ternary_1_label(array):
+    """
+    Test plotting a ternary chart with 1 label.
+    """
+    fig = Figure()
+    fig.ternary(
+        data=array,
+        region=[0, 100, 0, 100, 0, 100],
+        cmap="red,orange,yellow,green,blue,violet",
+        width="10c",
+        alabel="A",
+        frame=["bafg+lAir", "cafg+lLimestone", "aafg+lWater"],
+        style="c0.1c",
+        pen="thinnest",
+    )
+    return fig