Deprecate column cycling when plot() inputs have nonmatching shapes.

anntzer · anntzer · commit d9545b38a42e · 2017-11-24T11:54:27.000-08:00
Currently, something like

    plot([[0, 1, 2], [3, 4, 5]], [[0, 1], [2, 3]])

will plot column 0 of x vs column 0 of y; column 1 of x vs column 1 of
y; and column 2 of x vs column 1 of y (i.e. cycle among columns).  This
behavior appears to be error-prone to say the least.

Note that "normal" broadcasting (1-&gt;n) is not deprecated.
diff --git a/doc/api/api_changes/2017-11-24-AL.rst b/doc/api/api_changes/2017-11-24-AL.rst
@@ -0,0 +1,7 @@
+Deprecations
+````````````
+
+When given 2D inputs with non-matching numbers of columns, `~.pyplot.plot`
+currently cycles through the columns of the narrower input, until all the
+columns of the wider input have been plotted.  This behavior is deprecated; in
+the future, only broadcasting (1 column to *n* columns) will be performed.
diff --git a/lib/matplotlib/axes/_base.py b/lib/matplotlib/axes/_base.py
@@ -391,6 +391,9 @@ def _plot_args(self, tup, kwargs):
             func = self._makefill
 
         ncx, ncy = x.shape[1], y.shape[1]
+        if ncx > 1 and ncy > 1 and ncx != ncy:
+            cbook.warn_deprecated("2.2", "cycling among columns of inputs "
+                                  "with non-matching shapes is deprecated.")
         for j in xrange(max(ncx, ncy)):
             seg = func(x[:, j % ncx], y[:, j % ncy], kw, kwargs)
             ret.append(seg)
diff --git a/lib/matplotlib/tests/test_axes.py b/lib/matplotlib/tests/test_axes.py
@@ -26,7 +26,8 @@
 import matplotlib.patches as mpatches
 import matplotlib.colors as mcolors
 from numpy.testing import assert_allclose, assert_array_equal
-from matplotlib.cbook import IgnoredKeywordWarning
+from matplotlib.cbook import (
+    IgnoredKeywordWarning, MatplotlibDeprecationWarning)
 from matplotlib.cbook._backports import broadcast_to
 
 # Note: Some test cases are run twice: once normally and once with labeled data
@@ -5498,3 +5499,8 @@ def test_empty_errorbar_legend():
     ax.errorbar([], [], xerr=[], label='empty y')
     ax.errorbar([], [], yerr=[], label='empty x')
     ax.legend()
+
+
+def test_plot_columns_cycle_deprecation():
+    with pytest.warns(MatplotlibDeprecationWarning):
+        plt.plot(np.zeros((2, 2)), np.zeros((2, 3)))
