title: tkinter NavigationToolbar2TkAgg change [z] format for imshow
body: <p>I'm displaying an image using <code>imshow</code> inside a tkinter GUI and I have added a <code>NavigationToolbar2TkAgg</code> (python3.5). Very similarly to <a href="https://stackoverflow.com/questions/35750332/change-format-of-coordinate-text-in-status-bar/35750602">this question</a> (check it out for visuals), I want to change the formatting of the coordinates and z values (in particular, all my z values are between 0-10000, so I want to write them out instead of using scientific notation).</p>
  
  <p>For x and y this was pretty easy to do by changing the <code>format_coord</code> handle, but I can't seem to find anything to change the final bit <code>[xxx]</code>. I have tried <code>format_cursor_data</code>, but doesn't seem to be it either.</p>
  
  <p>Anyone know a solution?</p>
  
  <p>It seems that the <code>[z]</code> value is only shown for <code>imshow</code>, not for regular plots.</p>
  
  <p>Here goes a minimal code sample to reproduce the problem</p>
  
  <pre><code>import sys
  import numpy as np
  import matplotlib.pyplot as plt
  from matplotlib.figure import Figure
  
  import tkinter as tk
  from matplotlib.backends.backend_tkagg import FigureCanvasTkAgg,NavigationToolbar2TkAgg
  
  top = tk.Tk()
  
  fig = plt.figure()
  plt.imshow(np.array([[0,1],[1,2]]))
  
  ax = fig.gca()
  ax.format_coord = lambda x,y: "x:%4u, y:%4u" % (x,y)
  ax.format_cursor_data = lambda z: "Hello" % (z) # does nothing
  
  canvas = FigureCanvasTkAgg(fig,master=top)
  canvas.get_tk_widget().pack(side=tk.TOP, fill=tk.BOTH, expand=1)
  canvas.show()
  
  toolbar = NavigationToolbar2TkAgg(canvas, top)
  toolbar.update()
  
  top.mainloop()
  </code></pre>