title: invert radial scale on polar plot in matplotlib
body: <p>I'm using python 2.7.14 with matplotlib 2.1.1 on Debian buster. I want to create a polar plot with an inverted radial axis. My values on radial axis are mostly negative and I want lower values ploted at higher radius, as they represent a better outcome, and vice versa. Additionally I want to set the coordinate origin to a defined value. However matplotlib seems to cause problems by defining the limits with the second parameter undergoing the first one. There exist some solutions (maybe a bit outdated) to this problems <a href="https://stackoverflow.com/questions/43082637/reverse-radial-axes-of-matplotlib-polar-plot?utm_medium=organic&amp;utm_source=google_rich_qa&amp;utm_campaign=google_rich_qa">here</a> and <a href="https://stackoverflow.com/questions/12075001/set-radial-axis-on-matplotlib-polar-plots/12392209?utm_medium=organic&amp;utm_source=google_rich_qa&amp;utm_campaign=google_rich_qa">here</a>. But is there an more elegant way to invert the axis? My code looks like this:</p>
  
  <pre><code>#!/usr/bin/env python
  # -*- coding: utf-8 -*- 
  
  import matplotlib.pyplot as plt
  import numpy as np
  
  angle = [-90.,-45.,-30.,-20.,-13.,-9.,-6.,-4.,-2.,-1.,0.,1.,2.,4.,6.,9.,13.,20.,30.,45.,90.]
  tresh = [-16.1,-16.,-13.8,-12.4,-10.7,-9.4,-8.,-6.7,-6.,-6.5,-6.4,-6.7,-6.5,-7.4,-7.6,-10.1,-12.4,-14.3,-15.5,-16.9,-17.4]
  
  arc = np.deg2rad(angle)
  
  fig = plt.figure()
  ax = fig.add_subplot(111, polar=True)
  ax.plot(arc,tresh)
  
  ax.set_thetamin(-90)
  ax.set_thetamax(90)
  ax.set_theta_zero_location('N', offset=0)
  ax.set_theta_direction(-1)
  
  #ax.set_rorigin(-2)#This origin I want to set optionally.
  ax.set_ylim(-18,-5)
  
  #plt.show()
  fig.savefig('test.png')
  </code></pre>
  
  <p>The following picture explains want I want to do.
  <a href="https://i.stack.imgur.com/CPA9I.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/CPA9I.png" alt="enter image description here"></a></p>
  
  <p>Thank you for your help.</p>