Fix data ordering in panels when facetting

The bug appeared when facetting but there could be other
cases where it would show up. The real bug is in pandas,
how it sorts a dataframe by a categorical column.

fixes #26

doc/changelog.rst

@@ -12,6 +12,10 @@ v0.2.2
      is only set to the value ``'direct'`` if predictions will
      be made outside the data range.
 
+- Fixed bug where facetting led to a reordering of the data. This
+  would manifest as a bug for ``geoms`` where order was important.
+  (:issue:`26`)
+
 v0.2.1
 ------
 *(2017-06-22)*
@@ -33,7 +37,7 @@ v0.2.1
   the panels.
 
 - Fixed bug in :class:`~plotnine.stat_density` where changing the
-  x limits lead to an exception (:issue: `22`)
+  x limits lead to an exception (:issue:`22`)
 
 
 v0.2.0

plotnine/facets/facet_grid.py

@@ -142,6 +142,8 @@ def map(self, data, layout):
             keys = join_keys(facet_vals, layout, vars)
             data['PANEL'] = match(keys['x'], keys['y'], start=1)
 
+        data = data.sort_values('PANEL', kind='mergesort')
+
         # matching dtype and
         # the categories(panel numbers) for the data should be in the
         # same order as the panels. i.e the panels are the reference,
@@ -150,7 +152,7 @@ def map(self, data, layout):
             data['PANEL'],
             categories=layout['PANEL'].cat.categories,
             ordered=True)
-        data = data.sort_values('PANEL')
+
         data.reset_index(drop=True, inplace=True)
         return data
 

plotnine/facets/facet_wrap.py

@@ -137,13 +137,14 @@ def map(self, data, layout):
         # assign each point to a panel
         keys = join_keys(facet_vals, layout, self.vars)
         data['PANEL'] = match(keys['x'], keys['y'], start=1)
+        data = data.sort_values('PANEL', kind='mergesort')
 
         # matching dtype
         data['PANEL'] = pd.Categorical(
             data['PANEL'],
             categories=layout['PANEL'].cat.categories,
             ordered=True)
-        data = data.sort_values('PANEL')
+
         data.reset_index(drop=True, inplace=True)
         return data
 

plotnine/tests/test_geom_ribbon_area.py

@@ -4,7 +4,7 @@
 import pandas as pd
 
 from plotnine import (ggplot, aes, geom_area, geom_ribbon,
-                      scale_x_continuous, theme)
+                      facet_wrap, scale_x_continuous, theme)
 
 n = 4            # No. of ribbions in a vertical stack
 m = 100          # Points
@@ -57,3 +57,13 @@ def test_area_aesthetics():
          )
 
     assert p + _theme == 'area_aesthetics'
+
+
+def test_ribbon_facetting():
+    p = (ggplot(df, aes('x', ymin='ymin', ymax='ymax',
+                        fill='factor(z)')) +
+         geom_ribbon() +
+         facet_wrap('~ z')
+         )
+
+    assert p + _theme == 'ribbon_facetting'