title: Plot semi transparent contour plot over image file using matplotlib
body: <p>I'd like to plot a transparent contour plot over an image file in matplotlib/pyplot.</p>
  
  <p>Here's what I got so far...</p>
  
  <p>I have a 600x600 pixel square image file <code>test.png</code> that looks like so:</p>
  
  <p><a href="https://i.stack.imgur.com/YTHQg.png" rel="noreferrer"><img src="https://i.stack.imgur.com/YTHQg.png" alt="enter image description here"></a></p>
  
  <p>I would like to plot a contour plot over this image (having the image file be 'below' and a semi-transparent version of the contour plot overlaid) using matplotlib and pyplot. As a bonus, the image would be automatically scaled to fit within the current plotting boundaries. My example plotting script is as follows:</p>
  
  <pre><code>from matplotlib import pyplot
  from matplotlib.ticker import MultipleLocator, FormatStrFormatter
  from matplotlib.colors import BoundaryNorm
  from matplotlib.ticker import MaxNLocator
  from pylab import *
  import numpy as np
  import random
  
  # ----------------------------- #
  
  dx, dy = 500.0, 500.0
  y, x = np.mgrid[slice(-2500.0, 2500.0 + dy, dy),slice(-2500.0, 2500.0 + dx, dx)]
  
  z = []
  for i in x:
      z.append([])
      for j in y:
          z[-1].append(random.uniform(80.0,100.0))
  
  # ----------------------------- #
  
  plot_aspect = 1.2
  plot_height = 10.0
  plot_width = int(plot_height*plot_aspect)
  
  # ----------------------------- #
  
  pyplot.figure(figsize=(plot_width, plot_height), dpi=100)
  pyplot.subplots_adjust(left=0.10, right=1.00, top=0.90, bottom=0.06, hspace=0.30)
  subplot1 = pyplot.subplot(111)
  
  # ----------------------------- #
  
  cbar_max = 100.0
  cbar_min = 80.0
  cbar_step = 1.0
  cbar_num_colors = 200
  cbar_num_format = "%d"
  
  # ----------
  
  levels = MaxNLocator(nbins=cbar_num_colors).tick_values(cbar_min, cbar_max)
  cmap = pyplot.get_cmap('jet')
  norm = BoundaryNorm(levels, ncolors=cmap.N, clip=True)
  pp = pyplot.contourf(x,y,z,levels=levels,cmap=cmap)
  cbar = pyplot.colorbar(pp, orientation='vertical', ticks=np.arange(cbar_min, cbar_max+cbar_step, cbar_step), format=cbar_num_format)
  cbar.ax.set_ylabel('Color Scale [unit]', fontsize = 16, weight="bold")
  
  # ----------
  
  CS = pyplot.contour(x,y,z, alpha=0.5)
  
  # ----------
  
  majorLocator1   = MultipleLocator(500)
  majorFormatter1 = FormatStrFormatter('%d')
  minorLocator1   = MultipleLocator(250)
  
  subplot1.xaxis.set_major_locator(majorLocator1)
  subplot1.xaxis.set_major_formatter(majorFormatter1)
  subplot1.xaxis.set_minor_locator(minorLocator1)
  
  pyplot.xticks(fontsize = 16)
  pyplot.xlim(-2500.0,2500.0)
  
  # ----------
  
  majorLocator2   = MultipleLocator(500)
  majorFormatter2 = FormatStrFormatter('%d')
  minorLocator2   = MultipleLocator(250)
  
  subplot1.yaxis.set_major_locator(majorLocator2)
  subplot1.yaxis.set_major_formatter(majorFormatter2)
  subplot1.yaxis.set_minor_locator(minorLocator2)
  
  pyplot.yticks(fontsize = 16)
  pyplot.ylim(-2500.0,2500.0)
  
  # ----------
  
  subplot1.xaxis.grid()
  subplot1.yaxis.grid()
  
  # ----------
  
  subplot1.axes.set_aspect('equal')
  
  # ----------
  
  pyplot.suptitle('Main Title', fontsize = 24, weight="bold")
  
  # ----------
  
  pyplot.xlabel('X [m]', fontsize=16, weight="bold")
  pyplot.ylabel('Y [m]', fontsize=16, weight="bold")
  
  # ----------
  
  implot = subplot1.imshow( pyplot.imread('test.png') , interpolation='nearest', alpha=0.5)
  
  # ----------
  pyplot.show()
  #pyplot.savefig("tmp.png", dpi=100)
  pyplot.close()
  </code></pre>
  
  <p>...but I'm not getting the result I want... instead I just see the contour plot part. Something like:</p>
  
  <p><a href="https://i.stack.imgur.com/QXlPf.png" rel="noreferrer"><img src="https://i.stack.imgur.com/QXlPf.png" alt="enter image description here"></a></p>
  
  <p>What should I do in my code to get what I want?</p>