Subscribe to a topic in rqt plugin

edit

I want to subscribe to a very simple topic (/chatter) from within an rqt plugin but I cannot get it to work. After I boiled down the code it looks essentially like this:

void MyPlugin::initPlugin(qt_gui_cpp::PluginContext& context)
{
  mainWidget = new MainWindow();

  if (context.serialNumber() > 1)
  {
    mainWidget->setWindowTitle(mainWidget->windowTitle() + " (" + QString::number(context.serialNumber()) + ")");    
  }
  context.addWidget(mainWidget);

  // this is where want to subscribe
  sub = n.subscribe("chatter", 1000, &MyPlugin::chatterCallback, this);
}


// this is my callback function 
void MyPlugin::chatterCallback(const std_msgs::String::ConstPtr& msg)
{
  ROS_INFO("I heard: [%s]", msg->data.c_str());
}

Of course sub and n are declared in the header and the code compiles.

Only problem is: Nothing happens..