quiver: always recalculate when drawing; improve docstring; closes ma…

…tplotlib#3709, matplotlib#3817

The reason why the unconditional recalculation now seems to be
needed, but was not in v1.4.0, is unknown.

lib/matplotlib/quiver.py

@@ -87,12 +87,15 @@
 
 
   *angles*: [ 'uv' | 'xy' | array ]
-    With the default 'uv', the arrow aspect ratio is 1, so that
-    if *U*==*V* the angle of the arrow on the plot is 45 degrees
-    CCW from the *x*-axis.
+    With the default 'uv', the arrow axis aspect ratio is 1, so that
+    if *U*==*V* the orientation of the arrow on the plot is 45 degrees
+    CCW from the horizontal axis (positive to the right).
     With 'xy', the arrow points from (x,y) to (x+u, y+v).
+    Use this for plotting a gradient field, for example.
     Alternatively, arbitrary angles may be specified as an array
-    of values in degrees, CCW from the *x*-axis.
+    of values in degrees, CCW from the horizontal axis.
+    Note: inverting a data axis will correspondingly invert the
+    arrows *only* with `angles='xy'`.
 
   *scale*: [ *None* | float ]
     Data units per arrow length unit, e.g., m/s per plot width; a smaller
@@ -520,11 +523,9 @@ def get_datalim(self, transData):
     @allow_rasterization
     def draw(self, renderer):
         self._init()
-        if (self._new_UV or self.angles == 'xy'
-                or self.scale_units in ['x', 'y', 'xy']):
-            verts = self._make_verts(self.U, self.V)
-            self.set_verts(verts, closed=False)
-            self._new_UV = False
+        verts = self._make_verts(self.U, self.V)
+        self.set_verts(verts, closed=False)
+        self._new_UV = False
         mcollections.PolyCollection.draw(self, renderer)
 
     def set_UVC(self, U, V, C=None):

lib/matplotlib/tests/test_quiver.py

@@ -74,6 +74,15 @@ def test_quiver_with_key():
                                       'size': 'large'})
 
 
+@image_comparison(baseline_images=['quiver_single_test_image'],
+                  extensions=['png'], remove_text=True)
+def test_quiver_single():
+    fig, ax = plt.subplots()
+    ax.margins(0.1)
+
+    ax.quiver([1], [1], [2], [2])
+
+
 if __name__ == '__main__':
     import nose
     nose.runmodule()