title: How do I convert (or scale) axis values and redefine the tick frequency in matplotlib?
body: <p>I am displaying a jpg image (I rotate this by 90 degrees, if this is relevant) and of course
  the axes display the pixel coordinates. I would like to convert the axis so that instead of displaying the pixel number, it will display my unit of choice - be it radians, degrees, or in my case an astronomical coordinate. I know the conversion from pixel to (eg) degree. Here is a snippet of what my code looks like currently:</p>
  
  <pre><code>import matplotlib.pyplot as plt 
  import Image
  import matplotlib
  thumb = Image.open(self.image)
  thumb = thumb.rotate(90)
  dpi = plt.rcParams['figure.dpi']
  figsize = thumb.size[0]/dpi, thumb.size[1]/dpi
  fig = plt.figure(figsize=figsize)
  plt.imshow(thumb, origin='lower',aspect='equal')
  plt.show()
  </code></pre>
  
  <p>...so following on from this, can I take each value that matplotlib would print on the axis, and change/replace it with a string to output instead? I would want to do this for a specific coordinate format - eg, rather than an angle of 10.44 (degrees), I would like it to read 10 26' 24'' (ie, degrees, arcmins, arcsecs)</p>
  
  <p>Finally on this theme, I'd want control over the tick frequency, on the plot. Matplotlib might print the axis value every 50 pixels, but I'd really want it every (for example) degree.</p>
  
  <p>It sounds like I would like to define some kind of array with the pixel values and their converted values (degrees etc) that I want to be displayed, having control over the sampling frequency over the range xmin/xmax range.</p>
  
  <p>Are there any matplotlib experts on Stack Overflow? If so, thanks very much in advance for your help! To make this a more learning experience, I'd really appreciate being prodded in the direction of tutorials etc on this kind of matplotlib problem. I've found myself getting very confused with axes, axis, figures, artists etc!</p>
  
  <p>Cheers,</p>
  
  <p>Dave</p>