title: Update values in evenly spaced mesh with slider
body: <p>The problem I'm facing is that I'm unable to figure out how to update the figure when I increase/decrease values with my slider. </p>
  
  <p>I am creating an x, y mesh grid, then plotting it, and updating the values within my array to define a new mesh. I would like to be able to move the slider and see a finer/coarser grid and update the plot and axes.  </p>
  
  <p>Additionally, I would like the user to be able to move the grid lines interactively with the mouse. This is not included in example code. Does someone know how to implement this? </p>
  
  <p>There are similar questions that have been asked about updating values on a plot with a slider, but perhaps none address this specific problem; can someone help me figure out what I'm doing wrong? I realize I don't have anything that is redrawing the plot (though I have tested various things), but am unsure how to implement this, as M.plotGrid() creates the grid, so it may/may not be as easy as using fig.canvas.draw_idle(). </p>
  
  <p>Here is my code:</p>
  
  <pre><code>#Create simple evenly spaced mesh and allow slider to change values
  
  from SimPEG import Mesh, np
  import matplotlib.pyplot as plt
  from matplotlib.widgets import Slider, Button, RadioButtons
  
  hx = np.r_[0.1,0.1,0.1, 0.1, 0.1, 0.1, 0.1, 0.1, 0.1, 0.1]
  hy = np.r_[0.1,0.1,0.1, 0.1, 0.1, 0.1, 0.1, 0.1, 0.1, 0.1]
  
  M = Mesh.TensorMesh([hx, hy])
  M.plotGrid()
  
  #Define slider location, size, and color for adjusting mesh values
  axhx = plt.axes([0.25, 0.1, 0.65, 0.03])
  axhy = plt.axes([0.25, 0.15, 0.65, 0.03])
  xnode_inc = Slider(axhx, 'X mesh Node', 0.1, 1000.0, valinit=0)
  anode_inc = Slider(axhy, 'Y mesh Node', 0.1, 1000.0, valinit=0)
  
  def update(val):
      framehx = xnode_inc.val + hx
      framehy = ynode_inc.val + hy
      M = Mesh.TensorMesh([framehx, framehy])
      print M, framehx, framehy
      M.plotGrid()
  
  xnode_inc.on_changed(update)
  ynode_inc.on_changed(update)
  
  plt.show()
  </code></pre>