title: Matplotlib NavigationToolbar widget is not displayed in Tkinter GUI
body: <p>I'm designing a simple GUI with Tkinter composed of a ListBox (with Scrollbar)  and a canvas (FigureCanvasTkAgg) to plot. The ListBox is a collection of variables (time series) that are drawn as a x-y plot upon selection. The layout is schematically:</p>
  
  <pre><code>|---------------- Menu Bar --------------|
  |----------------------------------------|
  | |--- Frame 1-- | |------ Frame 2 ------|
  | |-- ListBox -- |^|                     |
  | |  Item 1      |-|                     |
  | |  Item 2      |-|       Canvas        |
  | |  Item 3      |-|                     |
  | |  Item 4      |-| ------------------- |
  | |  Item 5      |-|  Navigation toolbar |
  | |--------------|v|---------------------|
  |----------------------------------------| 
  </code></pre>
  
  <p>When the GUI is built, the Navigation Toolbar is absent and I have tried either with both pack() and grid() geometry managers. I apologize if the mistake is so evident but I'm not aware of what can be wrong. The minimal code is:</p>
  
  <pre><code>import matplotlib
  from matplotlib.figure import Figure
  from matplotlib.backends.backend_tkagg import FigureCanvasTkAgg, NavigationToolbar2TkAgg
  from tkinter import *
  
  class Plot_tool(Frame):
  
      def __init__(self, master=None):
          Frame.__init__(self, master)            
          self.master = master
          self.master.option_add('*tearOff', 'FALSE')
          self.master.title("Tool")
          self.options = [ i for i in range(50) ] # Some staff in ListBox
          self.pack(fill = BOTH, expand = 1)
          self.init_window()
  
      def init_window(self):
          menubar = Menu(self.master)
          self.master.config(menu = menubar)
  
          filemenu = Menu(menubar)
          filemenu.add_command(label = "Load")
          menubar.add_cascade(label = "File", menu = filemenu)
  
          # Frame 1 (ListBox and Scrollbar)
          framei = LabelFrame(self, text="frame", labelanchor="n")
          framei.pack(side="left", fill=Y)
  
          scrollbar = Scrollbar(framei)
          scrollbar.pack(side="right",fill=Y)
          self.items = Listbox(framei, yscrollcommand=scrollbar.set)
          self.items.pack(side="left", fill=Y)
          scrollbar.config(command = self.items.yview)
          self.items.insert(0, *self.options)
  
          # Frame 2 (Canvas and Navigation Toolbar)
          group2 = Frame(self)
          group2.pack(side="right")
  
          fig = Figure(figsize=(200,100))
          self.eje = fig.add_subplot(111)
          self.figura = FigureCanvasTkAgg(fig, master = group2)
          self.figura.get_tk_widget().pack()
          self.figura.show()
  
          toolbar = NavigationToolbar2TkAgg(self.figura, group2)
          toolbar.pack()
          toolbar.update()
  
  def on_closing(root):
      root.destroy()
      exit()
  
  def main():
      root = Tk()
      root.geometry("900x500")
      app = Plot_tool(root)
      root.protocol("WM_DELETE_WINDOW", lambda: on_closing(root))
      root.mainloop()
  
  if __name__ == '__main__':
      main()
  </code></pre>