Add barbs plot

docs/gallery_code/meteorology/plot_wind_barbs.py

@@ -0,0 +1,62 @@
+"""
+Plotting Wind Direction Using Barbs
+===================================
+
+This example demonstrates using barbs to plot wind speed contours and wind
+direction barbs from wind vector component input data. The vector components
+are co-located in space in this case.
+
+The magnitude of the wind in the original data is low and so doesn't illustrate
+the full range of barbs. The wind is scaled to simulate a storm that better
+illustrates the range of barbs that are available.
+"""
+
+import matplotlib.pyplot as plt
+
+import iris
+import iris.plot as iplt
+import iris.quickplot as qplt
+
+
+def main():
+    # Load the u and v components of wind from a pp file
+    infile = iris.sample_data_path("wind_speed_lake_victoria.pp")
+
+    uwind = iris.load_cube(infile, "x_wind")
+    vwind = iris.load_cube(infile, "y_wind")
+
+    uwind.convert_units("knot")
+    vwind.convert_units("knot")
+
+    # To illustrate the full range of barbs, scale the wind speed up to pretend
+    # that a storm is passing over
+    magnitude = (uwind ** 2 + vwind ** 2) ** 0.5
+    magnitude.convert_units("knot")
+    max_speed = magnitude.collapsed(
+        ("latitude", "longitude"), iris.analysis.MAX
+    ).data
+    max_desired = 65
+
+    uwind = uwind / max_speed * max_desired
+    vwind = vwind / max_speed * max_desired
+
+    # Create a cube containing the wind speed
+    windspeed = (uwind ** 2 + vwind ** 2) ** 0.5
+    windspeed.rename("windspeed")
+    windspeed.convert_units("knot")
+
+    plt.figure()
+
+    # Plot the wind speed as a contour plot
+    qplt.contourf(windspeed)
+
+    # Add wind barbs except for the outermost values which overhang the edge
+    # of the plot if left
+    iplt.barbs(uwind[1:-1, 1:-1], vwind[1:-1, 1:-1], pivot="middle", length=6)
+
+    plt.title("Wind speed during a simulated storm")
+    qplt.show()
+
+
+if __name__ == "__main__":
+    main()

docs/gallery_tests/test_plot_wind_barbs.py

@@ -0,0 +1,30 @@
+# Copyright Iris contributors
+#
+# This file is part of Iris and is released under the LGPL license.
+# See COPYING and COPYING.LESSER in the root of the repository for full
+# licensing details.
+
+# Import Iris tests first so that some things can be initialised before
+# importing anything else.
+import iris.tests as tests  # isort:skip
+
+from .gallerytest_util import (
+    add_gallery_to_path,
+    fail_any_deprecation_warnings,
+    show_replaced_by_check_graphic,
+)
+
+
+class TestWindBarbs(tests.GraphicsTest):
+    """Test the wind_barbs example code."""
+
+    def test_wind_barbs(self):
+        with fail_any_deprecation_warnings():
+            with add_gallery_to_path():
+                import plot_wind_barbs
+            with show_replaced_by_check_graphic(self):
+                plot_wind_barbs.main()
+
+
+if __name__ == "__main__":
+    tests.main()

docs/src/whatsnew/latest.rst

@@ -42,6 +42,9 @@ This document explains the changes made to Iris for this release
    the primary coordinate being plotted against is a vertical coordinate. E.g.
    ``iris.plot.plot(z_cube)`` will produce a z-vs-phenomenon plot, where before
    it would have produced a phenomenon-vs-z plot. (:pull:`3906`)
+#. `@jonseddon`_ added :meth:`iris.plot.barbs` to provide a convenient way to
+   use :func:`matplotlib.pyplot.barbs` with Iris cubes. A gallery example was
+   included to illustrate the new method's use. (:pull:`3710`)
 
 #. `@bjlittle`_ introduced :func:`iris.common.metadata.hexdigest` to the
    public API. Previously it was a private function introduced in ``v3.0.0``.

lib/iris/plot.py

@@ -1418,6 +1418,55 @@ def _vector_component_args(x_points, y_points, u_data, *args, **kwargs):
     return ((x_points, y_points, u_data, v_data), kwargs)
 
 
+def barbs(u_cube, v_cube, *args, **kwargs):
+    """
+    Draws a barb plot from two vector component cubes. Triangles, full-lines
+    and half-lines represent increments of 50, 10 and 5 respectively.
+
+    Args:
+
+    * u_cube, v_cube : (:class:`~iris.cube.Cube`)
+        u and v vector components.  Must have same shape and units.
+        If the cubes have geographic coordinates, the values are treated as
+        true distance differentials, e.g. windspeeds, and *not* map coordinate
+        vectors.  The components are aligned with the North and East of the
+        cube coordinate system.
+
+    .. Note::
+
+        At present, if u_cube and v_cube have geographic coordinates, then they
+        must be in a lat-lon coordinate system, though it may be a rotated one.
+        To transform wind values between coordinate systems, use
+        :func:`iris.analysis.cartography.rotate_grid_vectors`.
+        To transform coordinate grid points, you will need to create
+        2-dimensional arrays of x and y values.  These can be transformed with
+        :meth:`cartopy.crs.CRS.transform_points`.
+
+    Kwargs:
+
+    * coords: (list of :class:`~iris.coords.Coord` or string)
+        Coordinates or coordinate names. Use the given coordinates as the axes
+        for the plot. The order of the given coordinates indicates which axis
+        to use for each, where the first element is the horizontal
+        axis of the plot and the second element is the vertical axis
+        of the plot.
+
+    * axes: the :class:`matplotlib.axes.Axes` to use for drawing.
+        Defaults to the current axes if none provided.
+
+    See :func:`matplotlib.pyplot.barbs` for details of other valid
+    keyword arguments.
+
+    """
+    #
+    # TODO: check u + v cubes for compatibility.
+    #
+    kwargs["_v_data"] = v_cube.data
+    return _draw_2d_from_points(
+        "barbs", _vector_component_args, u_cube, *args, **kwargs
+    )
+
+
 def quiver(u_cube, v_cube, *args, **kwargs):
     """
     Draws an arrow plot from two vector component cubes.
@@ -1431,12 +1480,12 @@ def quiver(u_cube, v_cube, *args, **kwargs):
         vectors.  The components are aligned with the North and East of the
         cube coordinate system.
 
-    .. Note:
+    .. Note::
 
         At present, if u_cube and v_cube have geographic coordinates, then they
         must be in a lat-lon coordinate system, though it may be a rotated one.
         To transform wind values between coordinate systems, use
-        :func:`iris.analysis.cartography.rotate_vectors`.
+        :func:`iris.analysis.cartography.rotate_grid_vectors`.
         To transform coordinate grid points, you will need to create
         2-dimensional arrays of x and y values.  These can be transformed with
         :meth:`cartopy.crs.CRS.transform_points`.

lib/iris/tests/integration/plot/test_vector_plots.py

@@ -24,16 +24,16 @@
 if tests.MPL_AVAILABLE:
     import matplotlib.pyplot as plt
 
-    from iris.plot import quiver
+    from iris.plot import barbs, quiver
 
 
 @tests.skip_plot
 class MixinVectorPlotCases:
     """
-    Test examples mixin, used by separate quiver + streamplot classes.
+    Test examples mixin, used by separate barb, quiver + streamplot classes.
 
-    NOTE: at present for quiver only, as streamplot does not support arbitrary
-    coordinates.
+    NOTE: at present for barb and quiver only, as streamplot does not support
+    arbitrary coordinates.
 
     """
 
@@ -193,6 +193,34 @@ def test_circular_longitude(self):
         self.plot("circular", u_cube, v_cube, coords=("longitude", "latitude"))
 
 
+class TestBarbs(MixinVectorPlotCases, tests.GraphicsTest):
+    def setUp(self):
+        super().setUp()
+
+    @staticmethod
+    def _nonlatlon_xyuv():
+        # Increase the range of wind speeds used in the barbs test to test more
+        # barbs shapes than just circles
+        x, y, u, v = MixinVectorPlotCases._nonlatlon_xyuv()
+        scale_factor = 50
+        u *= scale_factor
+        v *= scale_factor
+        return x, y, u, v
+
+    @staticmethod
+    def _latlon_uv_cubes(grid_cube):
+        # Increase the range of wind speeds used in the barbs test to test all
+        # barbs shapes
+        u_cube, v_cube = MixinVectorPlotCases._latlon_uv_cubes(grid_cube)
+        scale_factor = 30
+        u_cube.data *= scale_factor
+        v_cube.data *= scale_factor
+        return u_cube, v_cube
+
+    def plot_function_to_test(self):
+        return barbs
+
+
 class TestQuiver(MixinVectorPlotCases, tests.GraphicsTest):
     def setUp(self):
         super().setUp()

lib/iris/tests/results/imagerepo.json

@@ -143,6 +143,10 @@
         "https://scitools.github.io/test-iris-imagehash/images/v4/fa8172d0847ecd2bc913939c36846c714933799cc3cc8727e67639f939996a58.png",
         "https://scitools.github.io/test-iris-imagehash/images/v4/fa8172c6857ecd38cb3392ce36c564311931d85ec64e9787719a39993c316e66.png"
     ],
+    "gallery_tests.test_plot_wind_barbs.TestWindBarbs.test_wind_barbs.0": [
+        "https://scitools.github.io/test-iris-imagehash/images/v4/e9e960e996169316c1fe9e96c29e36739e13c07c3d61c07f39a13921c07f3e21.png",
+        "https://scitools.github.io/test-iris-imagehash/images/v4/e9e161e996169316c1fe9e96c29e36739e13c07c3d61c07f39813929c07f3f01.png"
+    ],
     "gallery_tests.test_plot_wind_speed.TestWindSpeed.test_plot_wind_speed.0": [
         "https://scitools.github.io/test-iris-imagehash/images/v4/bcf924fb9306930ce12ccf97c73236b28ecec4cd3e29847b18e639e6c14f1a09.png",
         "https://scitools.github.io/test-iris-imagehash/images/v4/e9e960e996169306c1fe9e96c29e36739e13c06c3d61c07f39a139e1c07f3f01.png"
@@ -173,6 +177,21 @@
     "iris.tests.integration.plot.test_plot_2d_coords.Test2dContour.test_2d_coords_contour.0": [
         "https://scitools.github.io/test-iris-imagehash/images/v4/b4b2643ecb05cb43b0f23d80c53c4e1d3e5990eb1f81c19f2f983cb1c4ff3e42.png"
     ],
+    "iris.tests.integration.plot.test_vector_plots.TestBarbs.test_2d_plain_latlon.0": [
+        "https://scitools.github.io/test-iris-imagehash/images/v4/eb036726c47c9273918e6e2c6f216336787590eb969a165890ee6c676925b3b3.png"
+    ],
+    "iris.tests.integration.plot.test_vector_plots.TestBarbs.test_2d_plain_latlon_on_polar_map.0": [
+        "https://scitools.github.io/test-iris-imagehash/images/v4/e66d673c999031cd6667663398dc332c676364e798959336636660d933998666.png"
+    ],
+    "iris.tests.integration.plot.test_vector_plots.TestBarbs.test_2d_rotated_latlon.0": [
+        "https://scitools.github.io/test-iris-imagehash/images/v4/eba037a4c479c273b2963f2c6f6126966865d86f969e33c9b1706c26692793b0.png"
+    ],
+    "iris.tests.integration.plot.test_vector_plots.TestBarbs.test_non_latlon_1d_coords.0": [
+        "https://scitools.github.io/test-iris-imagehash/images/v4/a7ac334934d2e65c72596325b343338cb41c92d9c5b36f65330d379692ca6d6c.png"
+    ],
+    "iris.tests.integration.plot.test_vector_plots.TestBarbs.test_non_latlon_2d_coords.0": [
+        "https://scitools.github.io/test-iris-imagehash/images/v4/a7acb36134d2e676627963259343330cb43e92d9c5336e67330d379292ca6d6c.png"
+    ],
     "iris.tests.integration.plot.test_vector_plots.TestQuiver.test_2d_plain_latlon.0": [
         "https://scitools.github.io/test-iris-imagehash/images/v4/fb8d4f21c472b27e919d2e216f216b3178e69c7e961ab39a84696c616d245b94.png"
     ],