Cannot use rqt_plot to plot ardrone/navdata

I am new to ros. I just starting working with the tum_simulator, ardrone_autonomy and tum_ardrone packages. I am using the simulation in tum_simulator and not an actual drone. I would like to plot ardrone/navdata using rqt_plot. When I tried this I get a blank screen. The command I used was **$ rosrun rqt_plot rqt_plot /ardrone/navdata** When I run **$ rostopic echo /ardrone/navdata** I get the values. I don't understand why I can't plot the values that I obtain using rostopic echo.