factory for adding zoom callback to matplotlib graphs

simp_zoom.py

import matplotlib.pyplot as plt


def zoom_factory(ax,base_scale = 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 = 1/base_scale
        elif event.button == 'down':
            # deal with zoom out
            scale_factor = base_scale
        else:
            # deal with something that should never happen
            scale_factor = 1
            print event.button
        # set new limits
        ax.set_xlim([xdata - cur_xrange*scale_factor,
                     xdata + cur_xrange*scale_factor])
        ax.set_ylim([ydata - cur_yrange*scale_factor,
                     ydata + cur_yrange*scale_factor])
        ax.figure.canvas.draw_idle() # force re-draw the next time the GUI refreshes

    fig = ax.get_figure() # get the figure of interest
    # attach the call back
    fig.canvas.mpl_connect('scroll_event',zoom_fun)

    #return the function
    return zoom_fun

Thank you guys! I've found beneficial to implement 'event.step'. Posting the code.

Implementation of @giadang (commented on Jan 17, 2015) with event.step:

import numpy as np
#of course import matplotlib too... but that should be already part of your project

def zoom_factory(axis, base_scale=1e-3):
    """returns zooming functionality to axis"""
    def zoom_fun(event, ax, scale):
        """zoom when scrolling"""
        if event.inaxes == axis:
            scale_factor = np.power(scale,-event.step)
            xdata = event.xdata
            ydata = event.ydata
            x_left = xdata - ax.get_xlim()[0]
            x_right = ax.get_xlim()[1] - xdata
            y_top = ydata - ax.get_ylim()[0]
            y_bottom = ax.get_ylim()[1] - ydata

            ax.set_xlim([xdata - x_left * scale_factor,
                         xdata + x_right * scale_factor])
            ax.set_ylim([ydata - y_top * scale_factor,
                         ydata + y_bottom * scale_factor])
            ax.figure.canvas.draw()

    fig = axis.get_figure()
    fig.canvas.mpl_connect('scroll_event', lambda event: zoom_fun(
        event, axis, 1+base_scale))

My own implementation into Qt from mine project (stripped a bit) - shift selects the axis and also mods functionality:

import numpy as np
from PyQt5 import QtWidgets
#of course import matplotlib too... but that should be already part of your project

def zoom_factory(axis, base_scale=1e-3):
    """returns zooming functionality to axis"""
    def zoom_fun(event, ax, scale):
        """zoom when scrolling"""
        if event.inaxes == axis:
            scale_factor = np.power(scale,-event.step)
            if QtWidgets.QApplication.keyboardModifiers() !=\ 
                    QtCore.Qt.ShiftModifier:
                data = event.ydata
                new_top = data + (ax.get_ylim()[1] - data) \
                    * scale_factor
                ymin = -0.01
                axis.set_ylim([new_top * ymin, new_top])
            else:
                data = event.xdata
                x_left = data - ax.get_xlim()[0]
                x_right = ax.get_xlim()[1] - data
                ax.set_xlim([data - x_left * scale_factor,
                            data + x_right * scale_factor])
            ax.figure.canvas.draw()

    fig = axis.get_figure()
    fig.canvas.mpl_connect('scroll_event', lambda event: zoom_fun(
        event, axis, 1+base_scale))

There is another way of doing it as far as I know. By chance I came across the Axis.zoom method. I don't know if this is faster or a good way in general, but it works and is certainly less code:

    def zoom(self, event):
        if event.inaxes == self.ax:
            scale_factor = np.power(self.zoom_factor, -event.step)*event.step
            self.ax.get_xaxis().zoom(scale_factor)
            self.ax.get_yaxis().zoom(scale_factor)
            self.ax.invert_yaxis()
            self.canvas.draw_idle()

If you plot an image though, for some reason, you have to invert the y-axis again.

If you're using this in an environment with a toolbar and you want the home button to return to the original view after zooming:

then I found that you need to force the initial position to be pushed into the toolbars _nav_stack. When using the ipympl backend I did this using like so:

fig.canvas.toolbar.push_current()
zoom_factory(ax)

you could also add this to the zoom factory, but that may be risky as I'm not sure that fig.canvas.toolbaris guaranteed to exist.

Edit:
I just peeked around the forks of this gist and what I described with the toolbar was implemented in this one: https://gist.github.com/scott-vsi/522e756d636557ae8f1ef3cdb069cecd

that gist also does some nice checking inside of zoom_fun to also set the _nav_stack there if there are no views already pushed:

        toolbar = ax.get_figure().canvas.toolbar # only set the home state
        if toolbar._views.empty():
            toolbar.push_current()

@ianhi Can you please post an example code in which we zoom a plot. I'm a beginner with matplotlib and can't get to work the function right. Thanks!

This may help.
You do the plot, call zoom_factory, then show the plot:

fig = `plt.figure(...
graph = fig.add_subplot(111)
graph.plot(...
zf = zoom_factory(graph)
plt.show()

I've been using it quite happily but my code is a bit too specific but derived from simp_zoom.py

@mapfiable Interesting, I also did not know that existing until you pointed it out! The reason for the re-inversion is that you can invert the axis by setting the "min" to be greater than the "max" and inside of axis.zoom (which calls Locator.zoom) the inversion is discarded.

@tacaswell: do you think it is a good alternative? There is also Axis.pan, but I didn't manage to make it work properly.

Can we implement this code in jupyter Notebook?
If yes, please explain.

Yes, but you must use either %matplotlib notebook or %matplotlib widget which should "just work". If you are using %matplotlib inline it will not work (because that is not an interactive backend).

There's a small caveat to "just working" in the notebook - the widget backend doesn't have an option capture scroll events so you will end up scrolling the entire notebook while also zooming:

The workaround I used for this was to to use jupyterlab sidecar widget to display the plot as that won't have a scroll bar so it doesn't matter that the scroll input wasn't captured. Long term - I opened an issue and PR about this matplotlib/ipympl#222 that I think would fix this.
Also I think you will need to install the widget backend (https://github.com/matplotlib/ipympl#installation) as that isn't included with standard matplotlib

That is a fun bug! I left a review on your PR at ipympl.

Thanks!

For completeness for any future readers: The fix for scrolling in the notebook was merged and should be part of any release of ipympl after 0.5.6 (it is not in that release)

Hey, I tried to implement this in a wxPython Frame, but I keep getting a TypeError: zoom_fun() missing 1 required positional argument: 'event'. I know how to solve missing events in wxPython, but not in matplotlib. It should work because otherwise I wouldn't be getting the error on scrolling...
Edit: Turns out there is a work around, that only works if you're also pressing down the mouse wheel, not just scrolling with it...

@peroman200 Interesting, can you still reproduce that issue in mpl3.4?

License for your code?
Thanks

Note that this is implemented directly (using Control+scroll) in Matplotlib as of 3.11: matplotlib/matplotlib#30405