tacaswell / simp_zoom.py
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factory for adding zoom callback to matplotlib graphs

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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() # force re-draw

 

    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
jdreaver commented

Very nice!

One modification I made for my PyQt4 GUI: instead of using ax as a parameter, I pass in the parent canvas and say ax = canvas.ax on the first line. Then, instead of plt.draw(), I use canvas.draw(). However, your version works better for subplot zooming.

Owner
tacaswell commented

@jdreaver I didn't see your comment till recently. I am glad it was helpful! If you think your changes would be helpful for other people you should fork the gist.

I also tweaked it a bit to not call plt.draw().

giadang commented

IMHO, I think we don't have to calculate the cur_xrange and cur_yrange. Instead we can calculate the relative distance from the cursor position to the frame like this:

        # Get distance from the cursor to the edge of the figure frame
        x_left = xdata - cur_xlim[0]
        x_right = cur_xlim[1] - xdata
        y_top = ydata - cur_ylim[0]
        y_bottom = cur_ylim[1] - ydata

then set the new xlim, ylim like this:

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])

By this way, when zooming in and zooming out, the pixel which the cursor is pointing to won't be changed.

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