Blank TK Window

[Shamefaced]

Hi,
Any reason why a blank Tk() window opens up when I run my code:
Code:
for run in range(RUNS):
waittime = Monitor2()
checkouttime = Monitor2()
totaltimeinshop = Monitor2()
checkout_aisle = Simulation.Resource(AISLES)
Simulation.initialize()
cf = Customer_Market()
Simulation.activate(cf, cf.run(), 0.0)
Simulation.simulate(until=CLOSING)


Histo = waittime.histogram(low=0.0, high=80, nbins=40)
plt = SimPlot()
plt.plotLine(Histo, xlab='Time (min)',ylab='Qty Customers',
title="Wait Time",
color="red", width=2,
smooth='True')
plt.mainloop()

[woooee]

1. you no longer have a toplevel/root "plt" once you replace "plt"
with
    plt = SimPlot()
2. the SimPlot widget in never placed in Tkinter. This is usually
done with pack() or grid()

So start out with a Tkinter tutorial so you understand the basics.