diff --git a/grum/mainwin.py b/grum/mainwin.py
index e976ced..a2e4fd9 100644
--- a/grum/mainwin.py
+++ b/grum/mainwin.py
@@ -86,6 +86,7 @@ class MainWindow(QMainWindow):
             rst.start()
             rst.server.register_function(self.new_plot)
             rst.server.register_function(self.append_data)
+            rst.server.register_function(self.extend_data)
             rst.server.register_function(self.set_data)
 
         self.sig_make_new_plot.connect(self.on_make_new_plot)
@@ -119,6 +120,16 @@ class MainWindow(QMainWindow):
         desc.append(point)
         self.sync_item_and_plots(item)
 
+    def extend_data(self, name, data):
+        """
+        Extend the current data of the (existing) plot <name>.by <data>
+        The data is forwarded to the extend method of PlotDescription.
+        """
+        item = self.lst.get(name)
+        desc = item.value
+        desc.extend(data)
+        self.sync_item_and_plots(item)
+
     def set_data(self, name, data):
         """
         Set <data> as the data of the (existing) plot <name>.
diff --git a/grum/plotdesc.py b/grum/plotdesc.py
index eab0378..45832f3 100644
--- a/grum/plotdesc.py
+++ b/grum/plotdesc.py
@@ -24,6 +24,11 @@ class PlotDescription:
         self.xs.append(x)
         self.ys.append(y)
 
+    def extend(self, data):
+        xs, ys = data
+        self.xs.extend(xs)
+        self.ys.extend(ys)
+
 
     def make_plot(self, plotwidget, style):
         res = plotwidget.plot(self.xs, self.ys, name=self.name, **style)