title: Python - Superimpose 2 plots in subplot with matplotlib by using alpha channel
body: <p>I would like to superimpose 2 plots in one plot by using "alpha channel".
  I try to get the contour of the right figure on plot of left:</p>
  
  <p><img src="https://i.stack.imgur.com/MSw7a.png" alt="enter image description here"></p>
  
  <p>My code is :</p>
  
  <pre><code>plt.close()
  
  fig, axes = plt.subplots(nrows=1, ncols=3)
  
  plt.tight_layout(pad=0.05, w_pad=0.001, h_pad=2.0)
  ax1 = plt.subplot(131) # creates first axis
  ax1.set_xticks([0,2000,500,1000,1500])
  ax1.set_yticks([0,2000,500,1000,1500])
  ax1.tick_params(labelsize=8) 
  i1 = ax1.imshow(U,cmap='hot',extent=(X.min(),2000,Y.min(),2000), vmin=U.min(), vmax=U.max())
  cb1=plt.colorbar(i1,ax=ax1,ticks=[U.min(),(U.min()+U.max())/2., U.max()],fraction=0.046, pad=0.04,format='%.2f')
  cb1.ax.tick_params(labelsize=8)
  
  ax1.set_title("$ \mathrm{Ux_{mes} \/ (pix)}$", y=1.05, fontsize=12)
  ax2 = plt.subplot(132) # creates second axis
  ax2.set_xticks([0,2000,500,1000,1500])
  ax2.set_yticks([0,2000,500,1000,1500])
  i2=ax2.imshow(UU,cmap='hot',extent=(X.min(),2000,Y.min(),2000), vmin=U.min(), vmax=U.max())
  ax2.set_title("$\mathrm{Ux_{cal} \/ (pix)}$", y=1.05, fontsize=12)
  ax2.set_xticklabels([])
  ax2.set_yticklabels([])
  cb2=plt.colorbar(i2,ax=ax2,fraction=0.046, pad=0.04,ticks=[U.min(),(U.min()+U.max())/2.,U.max()],format='%.2f')
  cb2.ax.tick_params(labelsize=8)
  
  ax3 = plt.subplot(133) # creates first axis
  ax3.set_xticks([0,2000,500,1000,1500])
  ax3.set_yticks([0,2000,500,1000,1500])
  i3 = ax3.imshow(resU,cmap='hot',extent=(X.min(),2000,Y.min(),2000))
  ax3.set_title("$\mathrm{\mid Ux_{mes} - Ux_{cal} \mid \/ (pix)}$ ", y=1.05, fontsize=12)
  cb3=plt.colorbar(i3,ax=ax3,fraction=0.046, pad=0.04,ticks=[resU.min(), (resU.min()+resU.max())/2.,resU.max()],format='%.2f')
  ax3.set_xticklabels([])
  ax3.set_yticklabels([])
  cb3.ax.tick_params(labelsize=8)
  plt.gcf().tight_layout()
  
  plt.show()
  </code></pre>
  
  <p>I use the command below to get the contour and to add it :</p>
  
  <pre><code>[masquey,masquey] = np.gradient(masquey)
  plt.imshow(masquey2,cmap='hot',alpha=0.1)
  </code></pre>
  
  <p>but it doesnt work... i get that :</p>
  
  <p><img src="https://i.stack.imgur.com/8rkH8.png" alt="enter image description here"></p>