document api_changes

doc/api/api_changes/2015-03-14-SSH.rst

@@ -0,0 +1,16 @@
+``matplotlib.ticker.LinearLocator`` algorithm update
+```````````````````````````
+
+The ``matplotlib.ticker.LinearLocator`` is used to define the range and location
+of tickmarks of a plot when the user wants a exact number of ticks.
+``LinearLocator`` thus differs from the default locator ``MaxNLocator``, which
+does not necessarily return the exact user requested number of ticks.
+
+The view range algorithm in ``matplotlib.ticker.LinearLocator`` has been
+changed so that more convenient tick location are chosen. The new algorithm
+returns a plot view range that is a multiple of the user requested number of
+ticks. This ensures tick marks to be located at whole integers more
+constistently. For example, when both y-axis of a``twinx`` plot use
+``matplotlib.ticker.LinearLocator`` with the same number of ticks, the grids of
+both axis will be properly aligned at convenient tick locations.
+

lib/matplotlib/axes/_base.py

@@ -3780,7 +3780,8 @@ def twinx(self):
         create a twin of Axes for generating a plot with a sharex
         x-axis but independent y axis.  The y-axis of self will have
         ticks on left and the returned axes will have ticks on the
-        right.
+        right. To ensure tick marks of both axis align, see
+        :class:`~matplotlib.ticker.LinearLocator`
 
         .. note::
             For those who are 'picking' artists while using twinx, pick