diff --git a/tryton/gui/window/view_form/view/graph_gtk/bar.py b/tryton/gui/window/view_form/view/graph_gtk/bar.py
index 2e6b73e946..31b9acb8b6 100644
--- a/tryton/gui/window/view_form/view/graph_gtk/bar.py
+++ b/tryton/gui/window/view_form/view/graph_gtk/bar.py
@@ -1,4 +1,5 @@
 #This file is part of Tryton.  The COPYRIGHT file at the top level of this repository contains the full copyright notices and license terms.
+#This code is inspired by the pycha project (http://www.lorenzogil.com/projects/pycha/)
 from graph import Graph
 from tryton.common import hex2rgb, lighten
 import locale
diff --git a/tryton/gui/window/view_form/view/graph_gtk/graph.py b/tryton/gui/window/view_form/view/graph_gtk/graph.py
index 345c34f5ca..cb2b4522ab 100644
--- a/tryton/gui/window/view_form/view/graph_gtk/graph.py
+++ b/tryton/gui/window/view_form/view/graph_gtk/graph.py
@@ -1,4 +1,5 @@
 #This file is part of Tryton.  The COPYRIGHT file at the top level of this repository contains the full copyright notices and license terms.
+#This code is inspired by the pycha project (http://www.lorenzogil.com/projects/pycha/)
 import gtk
 from tryton.common import hex2rgb, generateColorscheme, DT_FORMAT, \
         COLOR_SCHEMES
diff --git a/tryton/gui/window/view_form/view/graph_gtk/line.py b/tryton/gui/window/view_form/view/graph_gtk/line.py
index 33e79362f5..591b3540b9 100644
--- a/tryton/gui/window/view_form/view/graph_gtk/line.py
+++ b/tryton/gui/window/view_form/view/graph_gtk/line.py
@@ -1,4 +1,5 @@
 #This file is part of Tryton.  The COPYRIGHT file at the top level of this repository contains the full copyright notices and license terms.
+#This code is inspired by the pycha project (http://www.lorenzogil.com/projects/pycha/)
 from graph import Graph
 from tryton.common import hex2rgb
 import locale
diff --git a/tryton/gui/window/view_form/view/graph_gtk/pie.py b/tryton/gui/window/view_form/view/graph_gtk/pie.py
index e23df12068..0a6f3bbd2b 100644
--- a/tryton/gui/window/view_form/view/graph_gtk/pie.py
+++ b/tryton/gui/window/view_form/view/graph_gtk/pie.py
@@ -1,4 +1,5 @@
 #This file is part of Tryton.  The COPYRIGHT file at the top level of this repository contains the full copyright notices and license terms.
+#This code is inspired by the pycha project (http://www.lorenzogil.com/projects/pycha/)
 from graph import Graph, Area
 import math
 import cairo