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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 librariesimport numpy as npimport timeimport matplotlib.pyplot as plt  # creating initial data values# of x and yx = np.linspace(0, 10, 100)y = np.sin(x)  # to run GUI event loopplt.ion()  # here we are creating sub plotsfigure, ax = plt.subplots(figsize=(10, 8))line1, = ax.plot(x, y)  # setting titleplt.title("Geeks For Geeks", fontsize=20)  # setting x-axis label and y-axis labelplt.xlabel("X-axis")plt.ylabel("Y-axis")  # Loopfor _ 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 piimport matplotlib.pyplot as pltimport numpy as npimport time  # genrating random data valuesx = np.linspace(1, 1000, 5000)y = np.random.randint(1, 1000, 5000)  # enable interactive modeplt.ion()  # creating subplot and figurefig = plt.figure()ax = fig.add_subplot(111)line1, = ax.plot(x, y)  # setting labelsplt.xlabel("X-axis")plt.ylabel("Y-axis")plt.title("Updating plot...")  # loopingfor _ 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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