added data setter

grum/plotdesc.py

@@ -14,6 +14,10 @@ class PlotDescription:
     def data(self):
         return (self.xs, self.ys)
 
+    @data.setter
+    def data(self, value):
+        self.xs, self.ys = value
+
 
     def append(self, xy):
         x, y = xy