title: python matplotlib widget doesn't work
body: <p>Now I'm solving a problem using euler method.
  I made it as follow but even though i move the slidr the graph doesn't change</p>
  
  <pre><code>from math import exp
  import numpy as np
  from matplotlib import pyplot as plt
  from matplotlib.widgets import Slider, Button, RadioButtons
  
  
  for i in range(1,n):
      Nlist.append(Nlist[i-1]+dNlist[i-1]*deltat)
      Qlist.append(Qlist[i-1]+Qlist[i-1]*deltat)
      dNlist.append(Qlist[i]/(1+Qlist[i])*(1-Nlist[i]/Nmax)*Nlist[i])
      tlist.append(t0+i*deltat)
  x=tlist
  y=Nlist
  fig=plt.figure()
  l=fig.add_subplot(111)
  fig.subplots_adjust(left=0.25, bottom=0.25)
  l.plot(x,y,linewidth=2,color="red")
  plt.xlabel("Value of t")
  plt.ylabel("Value of N")
  plt.title("Euler Method")
  
  
  axcolor = 'lightgoldenrodyellow'
  axQ0 = fig.add_axes([0.25, 0.15, 0.65, 0.03], axisbg=axcolor)
  axNmax = fig.add_axes([0.25, 0.1, 0.65, 0.03], axisbg=axcolor)
  axN0 = fig.add_axes([0.25, 0.05, 0.65, 0.03], axisbg=axcolor)
  
  sQ0 = Slider(axQ0, 'init_Temp', 0, 0.1, valinit=Q0)
  sNmax = Slider(axNmax, 'N_max', 1000, 100000, valinit=Nmax)
  sN0 = Slider(axN0, "init_N",1,10,valinit=N0)
  </code></pre>