Given 3 points on a 2D plane, design a function to do the following: determine whether 3 points lie on a straight line; if they do not lie on a straight line, draw a triangle with those points. (please import matplotlib) Note that the straight line or one of the triangle's edges may be perpendicular to the x-axis. Please take this possibility into account. The following shows the execution of your program.

Here is a Python function that takes in 3 points and determines whether they lie on a straight line or not. If they do not lie on a straight line, it uses Matplotlib to draw a triangle with those points:

import matplotlib.pyplot as plt

def plot_triangle(p1, p2, p3):
    # Check if the points lie on a straight line
    if (p2[1]-p1[1])*(p3[0]-p2[0]) == (p3[1]-p2[1])*(p2[0]-p1[0]):
        print('The points lie on a straight line.')
    else:
        # Create a figure and axis object
        fig, ax = plt.subplots()
        # Plot the points
        ax.plot(p1[0], p1[1], 'bo')
        ax.plot(p2[0], p2[1], 'bo')
        ax.plot(p3[0], p3[1], 'bo')
        # Plot the triangle
        ax.plot([p1[0], p2[0]], [p1[1], p2[1]], 'r-')
        ax.plot([p2[0], p3[0]], [p2[1], p3[1]], 'r-')
        ax.plot([p3[0], p1[0]], [p3[1], p1[1]], 'r-')
        # Set the x and y limits
        ax.set_xlim(min(p1[0], p2[0], p3[0])-1, max(p1[0], p2[0], p3[0])+1)
        ax.set_ylim(min(p1[1], p2[1], p3[1])-1, max(p1[1], p2[1], p3[1])+1)
        # Show the plot
        plt.show()

To use this function, simply call it with the 3 points as input:

p1 = (1, 1)
p2 = (2, 3)
p3 = (4, 2)
plot_triangle(p1, p2, p3)

This will plot a triangle with the points `(1, 1)`, `(2, 3)`, and `(4, 2)`. If the points lie on a straight line, the function will print "The points lie on a straight line." instead of plotting the triangle.