Allow passing an array as intensity for plot

pygmt/src/plot.py

@@ -218,6 +218,14 @@ def plot(self, x=None, y=None, data=None, sizes=None, direction=None, **kwargs):
             )
         extra_arrays.append(sizes)
 
+    if "I" in kwargs and is_nonstr_iter(kwargs["I"]):
+        if kind != "vectors":
+            raise GMTInvalidInput(
+                "Can't use arrays for intensity if data is matrix or file."
+            )
+        extra_arrays.append(kwargs["I"])
+        kwargs["I"] = ""
+
     if "t" in kwargs and is_nonstr_iter(kwargs["t"]):
         extra_arrays.append(kwargs["t"])
         kwargs["t"] = ""

pygmt/tests/test_plot.py

@@ -95,7 +95,8 @@ def test_plot_fail_no_data(data):
 
 def test_plot_fail_size_color(data):
     """
-    Should raise an exception if array sizes and color are used with matrix.
+    Should raise an exception if array color, sizes and intensity are used with
+    matrix.
     """
     fig = Figure()
     with pytest.raises(GMTInvalidInput):
@@ -117,6 +118,16 @@ def test_plot_fail_size_color(data):
             color="red",
             frame="afg",
         )
+    with pytest.raises(GMTInvalidInput):
+        fig.plot(
+            data=data,
+            region=region,
+            projection="X4i",
+            style="c0.2c",
+            color="red",
+            frame="afg",
+            intensity=data[:, 2],
+        )
 
 
 @check_figures_equal()
@@ -231,6 +242,42 @@ def test_plot_colors_sizes_proj(data, region):
     return fig
 
 
+@check_figures_equal()
+def test_plot_varying_intensity():
+    """
+    Plot the data with array-like intensity.
+    """
+    x = np.arange(1, 10)
+    y = np.arange(1, 10)
+    intensity = np.linspace(-1, 1, len(x))
+
+    fig_ref, fig_test = Figure(), Figure()
+    # Use single-character arguments for the reference image
+    with GMTTempFile() as tmpfile:
+        np.savetxt(tmpfile.name, np.c_[x, y, intensity], fmt="%s")
+        fig_ref.plot(
+            data=tmpfile.name,
+            R="0/10/0/10",
+            J="X4i",
+            B="",
+            S="c0.2c",
+            G="blue",
+            I="",
+        )
+
+    fig_test.plot(
+        x=x,
+        y=y,
+        region=[0, 10, 0, 10],
+        projection="X4i",
+        frame=True,
+        style="c0.2c",
+        color="blue",
+        intensity=intensity,
+    )
+    return fig_ref, fig_test
+
+
 @check_figures_equal()
 def test_plot_transparency():
     """