Fix butted path joins for geoms that use geom_path

Makefile

@@ -35,6 +35,7 @@ clean-test:
 	rm -fr .tox/
 	rm -f .coverage
 	rm -fr htmlcov/
+	rm -fr result_images/*
 
 lint:
 	flake8 plotnine

doc/changelog.rst

@@ -14,6 +14,10 @@ v0.1.1
 - Fixed exceptions when using :func:`pandas.pivot_table` for Pandas v0.20.0.
   The API was `fixed <http://pandas.pydata.org/pandas-docs/version/0.20/whatsnew.html#pivot-table-always-returns-a-dataframe>`_.
 
+- Fixed issues where lines/paths with segments that all belonged in the
+  same group had joins that in some cases were "butted".
+
+
 API Changes
 ***********
 

plotnine/geoms/geom_path.py

@@ -102,7 +102,7 @@ def draw_panel(self, data, panel_params, coord, ax, **params):
         cols = {'color', 'size', 'linetype', 'alpha', 'group'}
         cols = cols & set(data.columns)
         df = data.drop_duplicates(cols)
-        constant = len(df) == len(data['group'].unique())
+        constant = len(df) == data['group'].nunique()
         params['constant'] = constant
 
         if not constant:
@@ -117,7 +117,7 @@ def draw_panel(self, data, panel_params, coord, ax, **params):
     def draw_group(data, panel_params, coord, ax, **params):
         data = coord.transform(data, panel_params, munch=True)
         data['size'] *= SIZE_FACTOR
-        constant = params.pop('constant', False)
+        constant = params.pop('constant', data['group'].nunique() == 1)
 
         if not constant:
             _draw_segments(data, ax, **params)