title: How to use variables in ax.set_title and fig.savefig
body: <p>I goal is to plot a figure and give it a title and save it as png file.</p>
  
  <p>For example, I plot a figure first</p>
  
  <pre><code>from matplotlib import pyplot as plt
  import numpy as np
  from skimage import measure
  
  
  # Set up mesh
  n = 100
  
  x = np.linspace(-3,3,n)
  y = np.linspace(-3,3,n)
  z = np.linspace(-3,3,n)
  X, Y, Z =  np.meshgrid(x, y, z)
  
  # Create cardioid function
  def f_heart(x,y,z):
      F = 320 * ((-x**2 * z**3 -9*y**2 * z**3/80) +
                 (x**2 + 9*y**2/4 + z**2-1)**3)
      return F
  
  # Obtain value to at every point in mesh
  vol = f_heart(X,Y,Z)
  
  # Extract a 2D surface mesh from a 3D volume (F=0)
  verts, faces ,_ ,_ = measure.marching_cubes_lewiner(vol, 0, spacing=(0.1, 0.1, 0.1))
  
  
  # Create a 3D figure
  fig = plt.figure(figsize=(12,8))
  ax = fig.add_subplot(111, projection='3d')
  
  # Plot the surface
  ax.plot_trisurf(verts[:, 0], verts[:,1], faces, verts[:, 2],
                  cmap='Spectral', lw=1)
  
  # Change the angle of view and title
  ax.view_init(15, -15)
  </code></pre>
  
  <p>Now, I want to use a variable as a title to the plot and save it as png file.</p>
  
  <pre><code>#set the number of variable Θ
  Θ = 1
  
  #give the title
  ax.set_title("Θ = ", fontsize=25)
  
  
  plt.show()
  fig.savefig('D:/Θ = .png')
  </code></pre>
  
  <p>I hope this code can give me a plot with title Θ = 1 (or other value that I set for Θ), and save the plot at D:/ with a name Θ = 1.</p>
  
  <p>How can I edit my code to achieve this?
  Thank you!!!</p>