Matplotlib – Rectangle Selector

Matplotlib is a python library for visualization. It provides various widgets to make the visualization of data simple. There are some cases when there is a need of selection of particular region of a graph. For this interactivity, Matplotlib provides RectangleSelector widget. This widget helps to select a rectangular region of given axes. Also, it provides a way to perform actions according to the selection.

RectangleSelector(): Selects a rectangular region of given axes. 

Syntax: class matplotlib.widgets.RectangleSelector(ax, onselect, drawtype=’box’, minspanx=0, minspany=0, useblit=False, lineprops=None, rectprops=None, spancoords=’data’, button=None, maxdist=10, marker_props=None, interactive=False, state_modifier_keys=None)

Parameters:

• ax: A matplotlib.axes.Axes instance where widget is placed.
• onselect: Function to connect to the selection event. When the selection is completed, respective function is called. onselect function takes mouse click and mouse release events as arguments.
• drawtype: It specifies how to show selection. If “box”, then draws the full rectangle box. If “line”, then draws line of rectangle. If “none”, then draws nothing.  Default value is “box”.
• button:  It provides list of mouse buttons that can trigger rectangle selection. By default all buttons are allowed.
• maxdist: It is a distance in pixels within which the interactive tool handles can be activated. Default value is 10.
• interactive: It is a boolean value to specify whether to draw a set of handles for interaction with the widget.

Properties:

• center: Provides center of rectangle drawn
• corners: Provides corners of rectangle starting from lower left, moving clockwise.
• edge_centers: Provides midpoint of rectangle edges starting from left moving clockwise.
• extents: Returns (xmin, xmax, ymin, ymax).
• geometry: Returns an array containing the x and y co-ordinates of the four corners of the rectangle (starting and ending in the top left corner). Array shape is (2, 5).  all x coordinates can be obtained using RectangleSelector.geometry[1, :] and y coordinates using RectangleSelector.geometry[0, :].

Methods:

• draw_shape(self, extents): draws a rectangular shape using (xmin, xmax, ymin, ymax) values.

Example 1:

Following program demonstrates simple RectangleSelector used to select a region and zoom the selected area.

Python3

from matplotlib.widgets import RectangleSelector
import matplotlib.pyplot as plt

# Function to be executed after selection
def onselect_function(eclick, erelease):

    # Obtain (xmin, xmax, ymin, ymax) values
    # for rectangle selector box using extent attribute.
    extent = rect_selector.extents
    print("Extents: ", extent)

    # Zoom the selected part
    # Set xlim range for plot as xmin to xmax
    # of rectangle selector box.
    plt.xlim(extent[0], extent[1])

    # Set ylim range for plot as ymin to ymax
    # of rectangle selector box.
    plt.ylim(extent[2], extent[3])

# plot a line graph for data n
fig, ax = plt.subplots()
n = [4, 5, 6, 10, 12, 15, 20, 23, 24, 19]
ax.plot(n)

# Define a RectangleSelector at given axes ax.
# It calls a function named 'onselect_function'
# when the selection is completed.
# Rectangular box is drawn to show the selected region.
# Only left mouse button is allowed for doing selection.
rect_selector = RectangleSelector(
    ax, onselect_function, drawtype='box', button=[1])

# Display graph
plt.show()

Output: