title: Show grid lines over image in pyqtgraph
body: <p>I'm drawing an image in pyqtgraph, and I'd like to be able to see the grid lines. But the grid lines are always drawn underneath the image, so any black areas of the image obscure the grid. Here's a fairly minimal example:</p>
  
  <pre><code>import matplotlib  # necessary for interactive plots in pyqtgraph
  import pyqtgraph as pg
  import numpy as np
  n = 100000
  sigma_y = 1e-3
  sigma_x = 1e-3
  x0 = np.matrix([np.random.normal(0, sigma_x, n), np.random.normal(0, sigma_y, n)])
  bins = 30
  histogram, x_edges, y_edges = np.histogram2d(np.asarray(x0)[0], np.asarray(x0)[1], bins)
  x_range = x_edges[-1] - x_edges[0]
  y_range = y_edges[-1] - y_edges[0]
  
  imv = pg.ImageView(view=pg.PlotItem())
  imv.show()
  imv.setPredefinedGradient('thermal')
  imv.getView().showGrid(True, True)
  imv.setImage(histogram, pos=(x_edges[0], y_edges[0]), scale=(x_range / bins, y_range / bins))
  </code></pre>
  
  <p>Here's what I see (after zooming out a little). You can see that the black area of the image obscures the grid lines.</p>
  
  <p><a href="https://i.stack.imgur.com/SYb6B.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/SYb6B.png" alt="pyqtgraph image screenshot"></a></p>
  
  <p><strong>EDIT</strong>: it's possible in the GUI to change the black colour to transparent (not my first choice, but an OK workaround for now), so you can see the grid below the image. That works OK but I can't figure out how to do it in code. How do I get the lookup table out of the <code>ImageView</code> to modify it?</p>