Add the ability to inheritate a parametric style from another style

demo/moulinrouge/__init__.py

@@ -112,15 +112,17 @@ def create_app():
     @app.route("/all")
     @app.route("/all/<style>")
     @app.route("/all/<style>/<color>")
+    @app.route("/all/<style>/<color>/<base_style>")
     @app.route("/all/interpolate=<interpolate>")
-    def all(style='default', color=None, interpolate=None):
+    def all(style='default', color=None, interpolate=None, base_style=None):
         width, height = 600, 400
         data = random.randrange(1, 10)
         order = random.randrange(1, 10)
         if color is None:
             style = styles[style]
         else:
-            style = parametric_styles[style](color)
+            style = parametric_styles[style](
+                color, base_style=styles[base_style or 'default'])
         xy_series = _random(data, order)
         other_series = []
         for title, values in xy_series:

pygal/style.py

@@ -260,7 +260,8 @@ for op in ('lighten', 'darken', 'saturate', 'desaturate', 'rotate'):
     def get_style_for(op_name):
         operation = getattr(colors, op_name)
 
-        def parametric_style(color, step=10, max_=None, **kwargs):
+        def parametric_style(color, step=10, max_=None,
+                             base_style=None, **kwargs):
             if max_ is None:
                 violency = {
                     'darken': 50,
@@ -278,7 +279,12 @@ for op in ('lighten', 'darken', 'saturate', 'desaturate', 'rotate'):
                 return operation(color, percent)
 
             colors = list(map(modifier, range(0, max(2, step))))
-            return Style(colors=colors, **kwargs)
+            if base_style is None:
+                return Style(colors=colors, **kwargs)
+
+            base_style.__dict__.update(kwargs)
+            base_style._colors = colors
+            return base_style
 
         return parametric_style