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

anntzer · anntzer · commit 503af4b73207 · 2017-11-23T13:08:16.000-08:00
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)
