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How to update a plot on same figure during the loop?
  • Last Updated : 24 Feb, 2021

We can use matplotlib to update a plot on every iteration during the loop. With the help of matplotlib.pyplot.draw() function we can update the plot on the same figure during the loop.

Using matplotlib.pyplot.draw(): It is used to update a figure that has been changed. It will redraw the current figure.

Syntax: figure.canvas.draw()

Before this we use figure.ion() function to run a GUI event loop. Without using figure.ion() we may not be able to see the GUI plot.

Approach:

In the given example firstly we are importing all the necessary libraries that we are going to use. And then creating X and Y. X holds the values from 0 to 10 which evenly spaced into 100 values. e.g. we are creating values from 2 to 3 with evenly spaced 5 values (np.linspace(2, 3, 5)) It should output like these 5 values from 2 to 3 evenly spaced array([2 , 2.25, 2.5 , 2.75, 3 ]). After that we are initializing GUI using plt.ion() function, now we have to create a subplot, so we can plot X and Y values. After that we are running a for loop up to some iterations and creating a new_y values which hold our updating value then we are updating the values of X  and Y using set_xdata() and set_ydata(). And canvas.draw() will plot the updated values and canvas.flush_events() holds the GUI event till the UI events have been processed. This will run till the loop ends and values will be updated continuously.



Code:

Python3




# importing libraries
import numpy as np
import time
import matplotlib.pyplot as plt
  
# creating initial data values
# of x and y
x = np.linspace(0, 10, 100)
y = np.sin(x)
  
# to run GUI event loop
plt.ion()
  
# here we are creating sub plots
figure, ax = plt.subplots(figsize=(10, 8))
line1, = ax.plot(x, y)
  
# setting title
plt.title("Geeks For Geeks", fontsize=20)
  
# setting x-axis label and y-axis label
plt.xlabel("X-axis")
plt.ylabel("Y-axis")
  
# Loop
for _ in range(50):
    # creating new Y values
    new_y = np.sin(x-0.5*_)
  
    # updating data values
    line1.set_xdata(x)
    line1.set_ydata(new_y)
  
    # drawing updated values
    figure.canvas.draw()
  
    # This will run the GUI event
    # loop until all UI events
    # currently waiting have been processed
    figure.canvas.flush_events()
  
    time.sleep(0.1)

Output: 

Updating plot

Here, figure.canvas.flush_events()  is used to clear the old figure before plotting the updated figure.

Example 2: In this example code, we are updating the value of y in a loop using set_xdata() and redrawing the figure every time using canvas.draw().

Python3




from math import pi
import matplotlib.pyplot as plt
import numpy as np
import time
  
# genrating random data values
x = np.linspace(1, 1000, 5000)
y = np.random.randint(1, 1000, 5000)
  
# enable interactive mode
plt.ion()
  
# creating subplot and figure
fig = plt.figure()
ax = fig.add_subplot(111)
line1, = ax.plot(x, y)
  
# setting labels
plt.xlabel("X-axis")
plt.ylabel("Y-axis")
plt.title("Updating plot...")
  
# looping
for _ in range(50):
    
    # updating the value of x and y
    line1.set_xdata(x*_)
    line1.set_ydata(y)
  
    # re-drawing the figure
    fig.canvas.draw()
      
    # to flush the GUI events
    fig.canvas.flush_events()
    time.sleep(0.1)

Output:

Updating plot.

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