Show grid lines over image in pyqtgraph

Question

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:

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))

Here's what I see (after zooming out a little). You can see that the black area of the image obscures the grid lines.

EDIT : 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 ImageView to modify it?

Answer 1

Here is what I did.

glw = pyqtgraph.GraphicsLayoutWidget()
pw = glw.addPlot(0, 0)

# Fix Axes ticks and grid
for key in pw.axes:
    ax = pw.getAxis(key)

    # Set the grid opacity
    if grid_is_visible:
        ax.setGrid(grid_opacity * 255)
    else:
        ax.setGrid(False)

    # Fix Z value making the grid on top of the image
    ax.setZValue(1)

This did cause some other issue, I think. It may have been the context menu or it had to do with panning and zooming, because of how Qt was signaling the events. One axes got the event priority and prevented the event from propagating for the other axes to pan and zoom. I submitted a pull request to pyqtgraph, so that might not be an issue anymore. I can't remember what caused the problem though. It may work just fine for you. I was doing a lot of other things like changing the background color and changing the viewbox background color which caused some small issues.

As a note I also changed the image z value. You shouldn't have to though.

imv.setZValue(1)