Figure.meca: Fix the bug when passing a dict of scalar values to the spec parameter

pygmt/src/meca.py

@@ -283,6 +283,10 @@ def meca(
 
         # convert dict to pd.DataFrame so columns can be reordered
         if isinstance(spec, dict):
+            # convert values to ndarray so pandas doesn't complain about "all
+            # scalar values". See issue #2174 for details.
+            for key, value in spec.items():
+                spec[key] = np.atleast_1d(value)
             spec = pd.DataFrame(spec)
 
         # expected columns are:

pygmt/tests/test_meca.py

@@ -29,6 +29,25 @@ def test_meca_spec_dictionary():
     return fig
 
 
+@pytest.mark.mpl_image_compare(filename="test_meca_spec_dictionary.png")
+def test_meca_spec_dict_all_scalars():
+    """
+    Test supplying a dict with scalar values for all focal parameters.
+    """
+    fig = Figure()
+    # Right lateral strike slip focal mechanism
+    fig.meca(
+        spec=dict(
+            strike=0, dip=90, rake=0, magnitude=5, longitude=0, latitude=5, depth=0
+        ),
+        scale="2.5c",
+        region=[-1, 1, 4, 6],
+        projection="M14c",
+        frame=2,
+    )
+    return fig
+
+
 @pytest.mark.mpl_image_compare
 def test_meca_spec_dict_list():
     """