Event Handling in Matplotlib

Event based System usually is part of a recurring set of patterns. Often, they comprise of the following :

• An incoming event
• A mechanism that is used to respond to an event
• A looping construct (e.g. while loop, listener, and the message dispatch mechanism)

The events that are triggered are also a bit richer, including information like which Axes the event occurred in. The events also understand the Matplotlib coordinate system and report event locations in both pixel and data coordinates.

Syntax:

figure.canvas.mpl_connect( Event_name , callback function or method)

Parameters:

• Event_name: It could be any from the below table
• callback_function: It will define to handle the event.

List of events

Event_name	class	Description
button_press_event	MouseEvent	The mouse button is pressed
button_release_event	MouseEvent	The mouse button is released
draw_event	DrawEvent	The canvas draw occurs
key_press_event	KeyEvent	A key is pressed
key_release_event	KeyEvent	A key is released
motion_notify_event	MouseEvent	The motion of the mouse
pick_event	PickEvent	An object in the canvas is selected
resize_event	ResizeEvent	The figure canvas is resized
scroll_event	MouseEvent	The scroll wheel of the mouse is rolled
figure_enter_event	LocationEvent	The mouse enters a figure
axes_enter_event	LocationEvent	The mouse enters an axes object
axes_leave_event	LocationEvent	The mouse leaves an axes object
figure_leave_event	LocationEvent	The mouse leaves a figure

Note: That the classes are defined in matplotlib.backend_bases

MOUSE EVENT

• button_press_event: This event involves a mouse button press
• button_release_event: This event involves a mouse button release
• scroll_event: This event involves scrolling of the mouse
• motion_notify_event: This event involves a notification pertaining to the mouse movement

Example:

We have used the mpl_connect method, which must be called if you want to provide custom user interaction features along with your plots. This method will take two arguments:

• A string value for the event, which can be any of the values listed in the Event Name column of the preceding table
• A callback function or method

Python3

# importing the necessary modules
from IPython.display import Image
import matplotlib.pyplot as plt
import matplotlib as mpl
import numpy as np
import time
import sys
import random
import matplotlib
matplotlib.use('nbagg')


class MouseEvent:

    # initialization
    def __init__(self):
        (figure, axes) = plt.subplots()
        axes.set_aspect(1)
        figure.canvas.mpl_connect('button_press_event', self.press)
        figure.canvas.mpl_connect('button_release_event', self.release)

    # start event to show the plot
    def start(self):
        plt.show()  # display the plot

    # press event will keep the starting time when u
    # press mouse button
    def press(self, event):
        self.start_time = time.time()

    # release event will keep the track when you release
    # mouse button
    def release(self, event):
        self.end_time = time.time()
        self.draw_click(event)

    # drawing the plot
    def draw_click(self, event):
        # size = square (4 * duration of the time button
        # is keep pressed )
        size = 4 * (self.end_time - self.start_time) ** 2

        # create a point of size=0.002 where mouse button
        # clicked on the plot
        c1 = plt.Circle([event.xdata, event.ydata], 0.002,)

        # create a circle of radius 0.02*size
        c2 = plt.Circle([event.xdata, event.ydata], 0.02 * size, alpha=0.2)
        event.canvas.figure.gca().add_artist(c1)
        event.canvas.figure.gca().add_artist(c2)
        event.canvas.figure.show()


cbs = MouseEvent()

# start the event
cbs.start()

Output :

Example 2:

We are going to add color using the draw_click  method

Python3

def draw_click(self, event):

    # you can specified your own color list
    col = ['magneta', 'lavender', 'salmon', 'yellow', 'orange']
    cn = random.randint(0, 5)

    # size = square (4 * duration of the time button
    # is keep pressed )
    size = 4 * (self.end_time - self.start_time) ** 2

    # create a point of size=0.002 where mouse button
    # clicked on the plot
    c1 = plt.Circle([event.xdata, event.ydata], 0.002,)

    # create a circle of radius 0.02*size
    c2 = plt.Circle([event.xdata, event.ydata], 0.02 *
                    size, alpha=0.2, color=col[cn])
    event.canvas.figure.gca().add_artist(c1)
    event.canvas.figure.gca().add_artist(c2)
    event.canvas.figure.show()

Output: