FIX: try hard to ensure at least 2 ticks with 'auto'

efiring · efiring · commit e9252cad4048 · 2016-06-12T20:58:47.000-10:00
This requires falling back on a larger set of options for intervals between ticks. Closes #5784.
diff --git a/lib/matplotlib/ticker.py b/lib/matplotlib/ticker.py
@@ -1619,6 +1619,8 @@ def __init__(self, *args, **kwargs):
             will be removed.  If prune==None, no ticks will be removed.
 
         """
+        self._fine_steps = [1, 1.5, 2, 2.5, 3, 4, 5, 6, 8, 10]
+        self._fine_int_steps = [1, 2, 3, 4, 5, 6, 8, 10]
         if args:
             kwargs['nbins'] = args[0]
             if len(args) > 1:
@@ -1631,8 +1633,11 @@ def set_params(self, **kwargs):
         """Set parameters within this locator."""
         if 'nbins' in kwargs:
             self._nbins = kwargs['nbins']
-            if self._nbins != 'auto':
-                self._nbins = int(self._nbins)
+        if self._nbins != 'auto':
+            self._nbins = int(self._nbins)
+            self._steps2 = None
+        else:
+            self._steps2 = self._fine_steps
         if 'trim' in kwargs:
             warnings.warn(
                 "The 'trim' keyword has no effect since version 2.0.",
@@ -1650,7 +1655,8 @@ def set_params(self, **kwargs):
         if 'steps' in kwargs:
             steps = kwargs['steps']
             if steps is None:
-                self._steps = [1, 1.5, 2, 2.5, 3, 4, 5, 6, 8, 10]
+                self._steps = self._fine_steps
+                self._steps2 = None
             else:
                 if int(steps[-1]) != 10:
                     steps = list(steps)
@@ -1660,11 +1666,15 @@ def set_params(self, **kwargs):
             self._integer = kwargs['integer']
         if self._integer:
             self._steps = [n for n in self._steps if _divmod(n, 1)[1] < 0.001]
+            if self._steps2 is not None:
+                self._steps2 = self._fine_int_steps
 
-    def _raw_ticks(self, vmin, vmax):
+    def _raw_ticks(self, vmin, vmax, steps=None):
+        if steps is None:
+            steps = self._steps
         nbins = self._nbins
         if nbins == 'auto':
-            nbins = max(min(self.axis.get_tick_space(), 9), 1)
+            nbins = max(min(self.axis.get_tick_space(), 9), 2)
         scale, offset = scale_range(vmin, vmax, nbins)
         if self._integer:
             scale = max(1, scale)
@@ -1675,7 +1685,7 @@ def _raw_ticks(self, vmin, vmax):
         best_vmax = vmax
         best_vmin = vmin
 
-        for step in self._steps:
+        for step in steps:
             if step < scaled_raw_step:
                 continue
             step *= scale
@@ -1703,7 +1713,14 @@ def tick_values(self, vmin, vmax):
         vmin, vmax = mtransforms.nonsingular(
             vmin, vmax, expander=1e-13, tiny=1e-14)
         locs = self._raw_ticks(vmin, vmax)
+        nlocs = len(locs)
         prune = self._prune
+        if prune in ('lower', 'upper'):
+            nlocs -= 1
+        elif prune == 'both':
+            nlocs -= 2
+        if self._steps2 and nlocs < 2:
+            locs = self._raw_ticks(vmin, vmax, self._steps2)
         if prune == 'lower':
             locs = locs[1:]
         elif prune == 'upper':
