RectangleSelector Disappears on Zoom

Question

When I run this example and create a rectangular selection if I zoom or move the plot window around the selection disappears until I deselect the move or zoom tool and click on

Answer 1

Adding a callback for draw_events:

def mycallback(event):
    if RS.active:
        RS.update()
plt.connect('draw_event', mycallback)

makes the RectangleSelector persist after zooming or panning, and is compatible with useblit=True.

---

For example, using the code from the docs as a base:

from __future__ import print_function
from matplotlib.widgets import RectangleSelector
import numpy as np
import matplotlib.pyplot as plt
import matplotlib.widgets as widgets
import threading
import datetime as DT

def line_select_callback(eclick, erelease):
    'eclick and erelease are the press and release events'
    x1, y1 = eclick.xdata, eclick.ydata
    x2, y2 = erelease.xdata, erelease.ydata
    print("(%3.2f, %3.2f) --> (%3.2f, %3.2f)" % (x1, y1, x2, y2))
    print(" The button you used were: %s %s" % (eclick.button, erelease.button))

def toggle_selector(event):
    print(' Key pressed: {}'.format(event.key))
    if event.key in ['D', 'd'] and RS.active:
        print(' RectangleSelector deactivated.')
        RS.set_active(False)
        RS.set_visible(False)
        RS.update()
    if event.key in ['A', 'a'] and not RS.active:
        print(' RectangleSelector activated.')
        RS.set_active(True)
        RS.set_visible(True)
        RS.update()

def mycallback(event):
    if RS.active:
        # print('mycallback')
        RS.update()

# def persist_rect(newlims):
#     print('persist_rect')
#     RS.set_visible(True)
#     RS.update()

fig, ax = plt.subplots() 
# figtype = type(fig)
# figtype._draw = figtype.draw
# def mydraw(self, renderer):
#     print('figure.draw')
#     self._draw(renderer)
# figtype.draw = mydraw

N = 100000               
x = np.linspace(0.0, 10.0, N) 

RS = RectangleSelector(ax, line_select_callback,
                       drawtype='box', useblit=True,
                       button=[1, 3],  # don't use middle button
                       minspanx=5, minspany=5,
                       spancoords='pixels',
                       interactive=True)

plt.plot(x, +np.sin(.2*np.pi*x), lw=3.5, c='b', alpha=.7) 
plt.plot(x, +np.cos(.2*np.pi*x), lw=3.5, c='r', alpha=.5)
plt.plot(x, -np.sin(.2*np.pi*x), lw=3.5, c='g', alpha=.3)

plt.connect('key_press_event', toggle_selector)
plt.connect('draw_event', mycallback)
# ax.callbacks.connect('xlim_changed', persist_rect)
# ax.callbacks.connect('ylim_changed', persist_rect)

plt.show()

---

Why does mycallback work but persist_rect doesn't?

If you uncomment the commented-out statements above, you'll get some printed output which will look something as this:

figure.draw
mycallback
figure.draw
mycallback
(4.09, -0.53) --> (8.15, 0.38)
 The button you used were: 1 1
persist_rect
persist_rect
figure.draw
mycallback
 Key pressed: q

Notice that persist_rect gets called before figure.draw, while mycallback is called afterwards. figure.draw does not draw the RectangleSelection, but it does draw the Rectangle used for the background. So figure.draw obscures RectangleSelection. Thus persist_rect momentarily displays RectangleSelection, but it fails to persist. mycallback works because it is called after figure.draw.

Answer 2

In the source code for RectangularSelector the release method (line 2119) handles the visibility of the selector

def _release(self, event):   
"""on button release event"""
    if not self.interactive:
        self.to_draw.set_visible(False)

Subclass RectangleSelector to modify the release method

class visibleRectangleSelector(RectangleSelector):
    def release(self, event):
        super(visibleRectangleSelector, self).release(event)
        self.to_draw.set_visible(True)
        self.canvas.draw()   ##updates canvas for new selection

Sample Code using doc example

from __future__ import print_function
"""
Do a mouseclick somewhere, move the mouse to some destination, release
the button.  This class gives click- and release-events and also draws
a line or a box from the click-point to the actual mouseposition
(within the same axes) until the button is released.  Within the
method 'self.ignore()' it is checked whether the button from eventpress
and eventrelease are the same.

"""
from matplotlib.widgets import RectangleSelector
import numpy as np
import matplotlib.pyplot as plt


class visibleRectangleSelector(RectangleSelector):
    def release(self, event):
        super(visibleRectangleSelector, self).release(event)
        self.to_draw.set_visible(True)
        self.canvas.draw() 


def line_select_callback(eclick, erelease):
    'eclick and erelease are the press and release events'
    x1, y1 = eclick.xdata, eclick.ydata
    x2, y2 = erelease.xdata, erelease.ydata
    print("(%3.2f, %3.2f) --> (%3.2f, %3.2f)" % (x1, y1, x2, y2))
    print(" The button you used were: %s %s" % (eclick.button,
                                                erelease.button))


def toggle_selector(event):
    print(' Key pressed.')
    if event.key in ['Q', 'q'] and toggle_selector.RS.active:
        print(' RectangleSelector deactivated.')
        toggle_selector.RS.set_active(False)
    if event.key in ['A', 'a'] and not toggle_selector.RS.active:
        print(' RectangleSelector activated.')
        toggle_selector.RS.set_active(True)


fig, current_ax = plt.subplots()  # make a new plotting range
N = 100000  # If N is large one can see
x = np.linspace(0.0, 10.0, N)  # improvement by use blitting!

plt.plot(x, +np.sin(.2 * np.pi * x), lw=3.5, c='b', alpha=.7)  # plot something
plt.plot(x, +np.cos(.2 * np.pi * x), lw=3.5, c='r', alpha=.5)
plt.plot(x, -np.sin(.2 * np.pi * x), lw=3.5, c='g', alpha=.3)

print("\n      click  -->  release")

# drawtype is 'box' or 'line' or 'none'
toggle_selector.RS = RectangleSelector(
    current_ax,
    line_select_callback,
    drawtype='box',
    useblit=False,
    button=[1, 3],  # don't use middle button
    minspanx=5,
    minspany=5,
    spancoords='pixels',
    interactive=True)
plt.connect('key_press_event', toggle_selector)
plt.show()

Answer 3

If I understand correctly, the rectangle selector should stay visible throughout the process of panning or zooming. This could be achieved by not using blitting,

toggle_selector.RS = RectangleSelector(ax, ...,  useblit=False, ...)

A side effect of this is that the plotting may become slow depending on the complexity of the plot, as without blitting, the complete plot is continuously redrawn while using the rectangle selector.