Negative in stacked bars

out.py

@@ -12,9 +12,9 @@ with open('out-bar.svg', 'w') as f:
     f.write(bar.render())
 
 stackedbar = StackedBar()
-rng = [3, 32, 39, 12]
+rng = [3, -32, 39, 12]
 stackedbar.add('test1', rng)
-rng2 = [24, 8, 18, 12]
+rng2 = [24, -8, 18, 12]
 stackedbar.add('test2', rng2)
 rng3 = [6, 1, -10, 0]
 stackedbar.add('test3', rng3)

pygal/stackedbar.py

@@ -6,8 +6,11 @@ class StackedBar(BaseGraph):
 
     def _draw(self):
         transposed = zip(*[serie.values for serie in self.series])
-        vals = [sum(val) for val in transposed]
-        ymin, ymax = min(min(vals), 0), max(max(vals), 0)
+        positive_vals = [sum([val if val > 0 else 0 for val in vals])
+                           for vals in transposed]
+        negative_vals = [sum([val if val < 0 else 0 for val in vals])
+                           for vals in transposed]
+        ymin, ymax = min(min(negative_vals), 0), max(max(positive_vals), 0)
         length = len(self.series[0].values)
         x_pos = [x / float(length) for x in range(length + 1)
         ] if length > 1 else [0, 1]  # Center if only one value
@@ -28,7 +31,7 @@ class StackedBar(BaseGraph):
         self.svg.legend([serie.title for serie in self.series])
         self.svg.title()
 
-        stack_vals = [0] * length
+        stack_vals = [[0, 0] for i in range(length)]
         for serie in self.series:
             serie_node = self.svg.serie(serie.index)
             stack_vals = self.svg.stackbar(

pygal/svg.py

@@ -205,8 +205,8 @@ class Svg(object):
             height = self.view.y(0) - y
             x = x + padding
             y_txt = y + height / 2
-            shift = stack_vals[i]
-            stack_vals[i] += height
+            shift = stack_vals[i][int(height < 0)]
+            stack_vals[i][int(height < 0)] += height
             if height < 0:
                 y = y + height
                 height = -height