factory for adding zoom callback to matplotlib imshow - now available via https://github.com/ianhi/mpl-interactions

imshow_zoom.py

import matplotlib.pyplot as plt

def zoom_factory(ax,base_scale = 1.1):
    """
    parameters
    ----------
    ax : matplotlib axes object
        axis on which to implement scroll to zoom
    base_scale : float
        how much zoom on each tick of scroll wheel
 
    returns
    -------
    disconnect_zoom : function
        call this to disconnect the scroll listener
    """
    def limits_to_range(lim):
        return lim[1] - lim[0]
    
    fig = ax.get_figure() # get the figure of interest
    toolbar = fig.canvas.toolbar
    toolbar.push_current()
    orig_xlim = ax.get_xlim()
    orig_ylim = ax.get_ylim()
    orig_yrange = limits_to_range(orig_ylim)
    orig_xrange = limits_to_range(orig_xlim)
    orig_center = ((orig_xlim[0]+orig_xlim[1])/2, (orig_ylim[0]+orig_ylim[1])/2)

    def zoom_fun(event):
        # get the current x and y limits
        cur_xlim = ax.get_xlim()
        cur_ylim = ax.get_ylim()
        # set the range
        cur_xrange = (cur_xlim[1] - cur_xlim[0])*.5
        cur_yrange = (cur_ylim[1] - cur_ylim[0])*.5
        xdata = event.xdata # get event x location
        ydata = event.ydata # get event y location
        if event.button == 'up':
            # deal with zoom in
            scale_factor = base_scale
        elif event.button == 'down':
            # deal with zoom out
            scale_factor = 1/base_scale
        else:
            # deal with something that should never happen
            scale_factor = 1
        # set new limits
        new_xlim = [xdata - (xdata-cur_xlim[0]) / scale_factor,
                     xdata + (cur_xlim[1]-xdata) / scale_factor]
        new_ylim = [ydata - (ydata-cur_ylim[0]) / scale_factor,
                         ydata + (cur_ylim[1]-ydata) / scale_factor]
        new_yrange = limits_to_range(new_ylim)
        new_xrange = limits_to_range(new_xlim)

        if np.abs(new_yrange)>np.abs(orig_yrange):
            new_ylim = orig_center[1] -new_yrange/2 , orig_center[1] +new_yrange/2
        if np.abs(new_xrange)>np.abs(orig_xrange):
            new_xlim = orig_center[0] -new_xrange/2 , orig_center[0] +new_xrange/2
        ax.set_xlim(new_xlim)
        ax.set_ylim(new_ylim)

        toolbar.push_current()
        ax.figure.canvas.draw_idle() # force re-draw


    # attach the call back
    cid = fig.canvas.mpl_connect('scroll_event',zoom_fun)
    def disconnect_zoom():
        fig.canvas.mpl_disconnect(cid)    

    #return the disconnect function
    return disconnect_zoom

This fork adds on two things:

1. Pushes initial position to toolbar so the home button will work
2. If you zoom out past the initial bounds the image will be centered

Pairs well with: https://gist.github.com/ianhi/5f514c31f13cc28fbd167849867122b7