Integer levels and vmin/vmax (#1191)

Author: fmaussion

doc/whats-new.rst

@@ -70,6 +70,10 @@ Breaking changes
 - Coordinates used to index a dimension are now loaded eagerly into
   :py:class:`pandas.Index` objects, instead of loading the values lazily.
   By `Guido Imperiale <https://github.com/crusaderky>`_.
+- Automatic levels for 2d plots are now guaranteed to land on ``vmin`` and
+  ``vmax`` when these kwargs are explicitly provided (:issue:`1191`). The
+  automated level selection logic also slightly changed.
+  By `Fabien Maussion <https://github.com/fmaussion>`_.
 - xarray no longer supports python 3.3 or versions of dask prior to v0.9.0.
 
 Deprecations

xarray/plot/plot.py

@@ -358,7 +358,11 @@ def _plot2d(plotfunc):
         How to draw arrows extending the colorbar beyond its limits. If not
         provided, extend is inferred from vmin, vmax and the data limits.
     levels : int or list-like object, optional
-        Split the colormap (cmap) into discrete color intervals.
+        Split the colormap (cmap) into discrete color intervals. If an integer
+        is provided, "nice" levels are chosen based on the data range: this can
+        imply that the final number of levels is not exactly the expected one.
+        Setting ``vmin`` and/or ``vmax`` with ``levels=N`` is equivalent to
+        setting ``levels=np.linspace(vmin, vmax, N)``.
     infer_intervals : bool, optional
         Only applies to pcolormesh. If True, the coordinate intervals are
         passed to pcolormesh. If False, the original coordinates are used

xarray/plot/utils.py

@@ -7,6 +7,7 @@
 import pandas as pd
 
 from ..core.pycompat import basestring
+from ..core.utils import is_scalar
 
 
 def _load_default_cmap(fname='default_colormap.csv'):
@@ -139,6 +140,9 @@ def _determine_cmap_params(plot_data, vmin=None, vmax=None, cmap=None,
     if (vmin is not None) and (vmax is not None):
         possibly_divergent = False
 
+    # Setting vmin or vmax implies linspaced levels
+    user_minmax = (vmin is not None) or (vmax is not None)
+
     # vlim might be computed below
     vlim = None
 
@@ -187,9 +191,13 @@ def _determine_cmap_params(plot_data, vmin=None, vmax=None, cmap=None,
 
     # Handle discrete levels
     if levels is not None:
-        if isinstance(levels, int):
-            ticker = mpl.ticker.MaxNLocator(levels)
-            levels = ticker.tick_values(vmin, vmax)
+        if is_scalar(levels):
+            if user_minmax or levels == 1:
+                levels = np.linspace(vmin, vmax, levels)
+            else:
+                # N in MaxNLocator refers to bins, not ticks
+                ticker = mpl.ticker.MaxNLocator(levels-1)
+                levels = ticker.tick_values(vmin, vmax)
         vmin, vmax = levels[0], levels[-1]
 
     if extend is None:

xarray/test/test_plot.py

@@ -2,6 +2,16 @@
 from __future__ import division
 from __future__ import print_function
 
+# import mpl and change the backend before other mpl imports
+try:
+    import matplotlib as mpl
+    # Using a different backend makes Travis CI work
+    mpl.use('Agg')
+    # Order of imports is important here.
+    import matplotlib.pyplot as plt
+except ImportError:
+    pass
+
 import inspect
 
 import numpy as np
@@ -17,15 +27,6 @@
 
 from . import TestCase, requires_matplotlib
 
-try:
-    import matplotlib as mpl
-    # Using a different backend makes Travis CI work.
-    mpl.use('Agg')
-    # Order of imports is important here.
-    import matplotlib.pyplot as plt
-except ImportError:
-    pass
-
 
 def text_in_fig():
     '''
@@ -322,20 +323,41 @@ def test_center(self):
 
     def test_integer_levels(self):
         data = self.data + 1
+
+        # default is to cover full data range but with no guarantee on Nlevels
+        for level in np.arange(2, 10, dtype=int):
+            cmap_params = _determine_cmap_params(data, levels=level)
+            self.assertEqual(cmap_params['vmin'], cmap_params['levels'][0])
+            self.assertEqual(cmap_params['vmax'], cmap_params['levels'][-1])
+            self.assertEqual(cmap_params['extend'], 'neither')
+
+        # with min max we are more strict
         cmap_params = _determine_cmap_params(data, levels=5, vmin=0, vmax=5,
                                              cmap='Blues')
+        self.assertEqual(cmap_params['vmin'], 0)
+        self.assertEqual(cmap_params['vmax'], 5)
         self.assertEqual(cmap_params['vmin'], cmap_params['levels'][0])
         self.assertEqual(cmap_params['vmax'], cmap_params['levels'][-1])
         self.assertEqual(cmap_params['cmap'].name, 'Blues')
         self.assertEqual(cmap_params['extend'], 'neither')
-        self.assertEqual(cmap_params['cmap'].N, 5)
-        self.assertEqual(cmap_params['norm'].N, 6)
+        self.assertEqual(cmap_params['cmap'].N, 4)
+        self.assertEqual(cmap_params['norm'].N, 5)
 
         cmap_params = _determine_cmap_params(data, levels=5,
                                              vmin=0.5, vmax=1.5)
         self.assertEqual(cmap_params['cmap'].name, 'viridis')
         self.assertEqual(cmap_params['extend'], 'max')
 
+        cmap_params = _determine_cmap_params(data, levels=5,
+                                             vmin=1.5)
+        self.assertEqual(cmap_params['cmap'].name, 'viridis')
+        self.assertEqual(cmap_params['extend'], 'min')
+
+        cmap_params = _determine_cmap_params(data, levels=5,
+                                             vmin=1.3, vmax=1.5)
+        self.assertEqual(cmap_params['cmap'].name, 'viridis')
+        self.assertEqual(cmap_params['extend'], 'both')
+
     def test_list_levels(self):
         data = self.data + 1