Version of imshow which shows the value under the cursor. Modified from poppy: http://www.stsci.edu/~mperrin/software/poppy/_modules/poppy/utils.html

imshowz.py

def imshowz(image, ax=None,  *args, **kwargs):
    """ wrapper for matplotlib imshow that displays the value under the cursor position

    Wrapper for pyplot.imshow that sets up a custom mouseover display formatter
    so that mouse motions over the image are labeled in the status bar with
    pixel numerical value as well as X and Y coords.

    Why this behavior isn't the matplotlib default, I have no idea...
    """
    if ax is None: ax = plt.gca()
    myax = ax.imshow(image, cmap='gray', origin='lower', interpolation='none', *args, **kwargs)
    aximage = ax.images[0].properties()['array']
    # need to account for half pixel offset of array coordinates for mouseover relative to pixel center,
    # so that the whole pixel from e.g. ( 1.5, 1.5) to (2.5, 2.5) is labeled with the coordinates of pixel (2,2)


    # We use the extent and implementation to map back from the data coord to pixel coord
    # There is probably an easier way to do this...
    imext = ax.images[0].get_extent()  # returns [-X, X, -Y, Y]
    imsize = ax.images[0].get_size()   # returns [sY, sX]g
    # map data coords back to pixel coords:
    #pixx = (x - imext[0])/(imext[1]-imext[0])*imsize[1]
    #pixy = (y - imext[2])/(imext[3]-imext[2])*imsize[0]
    # and be sure to clip appropriatedly to avoid array bounds errors
    report_pixel = lambda x, y : "(%6.3f, %6.3f)     %-12.6g" %         (x,y,   aximage[np.floor( (y - imext[2])/(imext[3]-imext[2])*imsize[0]  ).clip(0,imsize[0]-1),                        np.floor( (x - imext[0])/(imext[1]-imext[0])*imsize[1]  ).clip(0,imsize[1]-1)])

        #(x,y, aximage[np.floor(y+0.5),np.floor(x+0.5)])   # this works for regular pixels w/out an explicit extent= call
    ax.format_coord = report_pixel

    return ax