Animating scatter points with matplotlib

Question

I created a script that animates a line and scatters points, here is the script:

import numpy as np
import matplotlib.pyplot as plt
import matplotlib.animation as animation

fig, ax = plt.subplots(figsize=(16,8))
ax.set(xlim=(0,104), ylim=(0,68))

x_start, y_start = (50, 35)
x_end, y_end = (90, 45)

x = np.linspace(x_start, x_end, 20)
y = np.linspace(y_start, y_end, 20)

## the first scatter point
sc_1 = ax.scatter([], [], color="crimson", zorder=4, s=150, alpha=0.7, edgecolor="w")

## the line
line, = ax.plot([], [], color="black", zorder=4)

## the last scatter point
sc_2 = ax.scatter([], [], color="crimson", zorder=4, s=150, alpha=0.7, edgecolor="w")

## titles
title = ax.text(50, 65, "", bbox={'facecolor':'w', 'alpha':0.5, 'pad':5}, ha="center")

## scatter point which will follow the line
sc_3 = ax.scatter([], [], color="crimson", zorder=4, s=150, edgecolor="w")

def animate(i):
    if i == 1:
       sc_1.set_offsets([x_start, y_start])
       title.set_text("Action 001")

    if i == 2:
        plt.pause(0.5)
        title.set_text("Action 002")

    ## plot line
    line.set_data(x[:i], y[:i])

    ## plot scatter point that will follow the line
    sc_3.set_offsets(np.c_[x[:i], y[:i]])
    
    ## plot scatter point
    if i > len(x):

        plt.pause(0.2)
        title.set_text("Action 003")
        sc_2.set_offsets([x_end, y_end])

    return sc_1, line, sc_2, title, sc_3

ani = animation.FuncAnimation(  
    fig=fig, func=animate, interval=50, blit=True)  

ani.save("a.gif", writer="imagemagick")

plt.show()

It is giving the following output:

But what I want is while animating the scatter point using sc_3 the program should remove the last iteration of scatter point produced by sc_3, i.e. scatter point should not be plotted over the whole line, just a scatter point which will follow the line from start to the end.

The final output is like: scatter point at the start of the line and then a scatter point will follow the line(removing its last state position) and a final scatter point at the end of the line

What should be added in the program to get the desired output?

user_9214357 · Answer

I have slightly edited your code as following:

import numpy as np
import matplotlib.pyplot as plt
import matplotlib.animation as animation

fig, ax = plt.subplots(figsize = (16, 8))
ax.set(xlim = (0, 104), ylim = (0, 68))

x_start, y_start = (50, 35)
x_end, y_end = (90, 45)

N = 20
x = np.linspace(x_start, x_end, N)
y = np.linspace(y_start, y_end, N)

## the first scatter point
sc_1 = ax.scatter([], [], color = "crimson", zorder = 4, s = 150, alpha = 0.7, edgecolor = "w")

## the line
line, = ax.plot([], [], color = "black", zorder = 4)

## the last scatter point
sc_2 = ax.scatter([], [], color = "crimson", zorder = 4, s = 150, alpha = 0.7, edgecolor = "w")

## titles
title = ax.text(50, 65, "", bbox = {'facecolor': 'w', 'alpha': 0.5, 'pad': 5}, ha = "center")

## scatter point which will follow the line
sc_3 = ax.scatter([], [], color = "crimson", zorder = 4, s = 150, edgecolor = "w")

def animate(i):

    if i == 1:
        sc_1.set_offsets([x_start, y_start])
        title.set_text("Action 001")

    if i == 2:
        plt.pause(0.5)
        title.set_text("Action 002")

    ## plot line and scatter point that will follow the line
    if i >= 2:
        line.set_data(x[:i + 1], y[:i + 1])
        sc_3.set_offsets([x[i], y[i]])

    # plot scatter point
    if i == N - 1:
        plt.pause(0.2)
        title.set_text("Action 003")
        sc_2.set_offsets([x_end, y_end])

    return sc_1, line, sc_2, title, sc_3,


ani = animation.FuncAnimation(fig = fig, func = animate, frames = N, interval = 50, blit = True, repeat = False)
ani.save("a.gif", writer = "imagemagick")

plt.show()

The output animation is:

user_14293552 · Answer

I tried this:

    ## plot scatter point that will follow the line
    sc_3.set_offsets(np.c_[x[i], y[i]])

The end result is that it retains a single red point, but animates a single point moving along with the line.

Edit: the first red point is given by sc_1, which I assume you want to retain.

Animating scatter points with matplotlib

2 Answers