3. Matplotlib plot multiple lines

Matplotlib can be used to draw a graph with multiple lines.

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3.1. Simple Multiple line plot

This is a basic plot for the equations y = 2x - 2, y = -3x + 3, y = (x - 1)², y = 0.4(x - 1)³.

It has X and Y axes.

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3.2. Python code

The python code is below.

The code is commented to indicate what each part is doing.

import numpy as np
import matplotlib.pyplot as plt


def plot_line_graph(title, equations, min_x, max_x):
    """Plots line graphs of given equations.

    Args:
        title (str): The title of the graph.
        equations (list of str): A list of strings representing the equations to be plotted.
        min_x (int): The minimum value of x to plot.
        max_x (int): The maximum value of x to plot.
        
    Returns:
        None
    """
    # Size the Graph
    fig = plt.figure(figsize=(5,5), dpi=100)
    # Define the range of x values
    x = np.linspace(min_x, max_x, 100)
    # plot each equation
    for equation in equations:
        # Calculate the corresponding y values
        y = eval(equation)
        # Plot the line graph
        plt.plot(x, y)
    # Add x, y axis lines through the origin
    plt.axhline(0, color="gray", linestyle="-")
    plt.axvline(0, color="gray", linestyle="-")
    # Add a title (specify font parameters with fontdict)
    plt.title(title, fontdict={"fontname": "Lucida Sans", "fontsize": 16})
    # Show plot
    plt.show()


def main():
    plot_line_graph("Multiple lines", ["2 * x - 2", "-3 * x + 3", "(x-1)**2", "0.4*(x-1)**3"], -1, 3)


if __name__ == '__main__':
    main()

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3.3. Straight line graph with parabola

This is a plot for the equation y = 2x + 1 and y = (x - 1)².

This plot has more customizations added.

It has code to save the plot as a png file.

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3.4. Python code

The python code is below.

The code is commented to indicate what each part is doing.

from pathlib import Path
import numpy as np
import matplotlib.pyplot as plt


def plot_line_graph(title, equations, labels):
    """Plots line graphs of given equations and labels them.

    Args:
        title (str): The title of the graph.
        equations (list of str): A list of strings representing the equations to be plotted.
        labels (list of str): A list of strings representing the labels for each equation.

    Returns:
        None
    """
    # Define the range of x values
    x = np.linspace(-1, 5, 100)
    # Resize the Graph (dpi specifies pixels per inch. When saving probably should use 300 if possible)
    fig = plt.figure(figsize=(8, 8), dpi=100)
    # Adjust subplot parameters to make room for the legend
    fig.subplots_adjust(right=0.75)
    # set initial y_min and y_max that will be used for y ticks
    y_min = 0
    y_max = 0
    for i, (equation, label) in enumerate(zip(equations, labels)):
        # Calculate the corresponding y values
        y = eval(equation)
        # since y is an array, need to do min(y) first before combining with y_min
        y_min = int(min(min(y), y_min))
        y_max = int(max(max(y), y_max))
        # use default colours C0, C1...C9
        color = f'C{i}'
        # Plot the line graph
        plt.plot(x, y, label=label, color=color)
        # Create a lambda function from the equation string to allow use of variable xi
        f = eval("lambda x: " + equation)
        # Label each point on the graph for x = -1 to 5 as integers
        for j in range(-1, 6):
            xi = j
            yi = f(xi)
            plt.plot(xi, yi, "o", color=color)
            # test to see if value is an integer so y coordinates can avoid using decimals
            if int(yi) == yi:
                plt.text(xi+0.1, yi-0.25, f"({xi}, {int(yi)})", fontsize=10)
            else:
                plt.text(xi+0.1, yi-0.25, f"({xi}, {yi:.2f})", fontsize=10)
    # Calculate the corresponding y values for both lines
    y1 = eval(equations[0])
    y2 = eval(equations[1])            
    # Fill the area between the two lines
    plt.fill_between(x, y1, y2, where=(y1 > y2), interpolate=True, color='green', alpha=0.2, label="Line above")
    plt.fill_between(x, y1, y2, where=(y1 <= y2), interpolate=True, color='red', alpha=0.2, label="Curve above")
    # Add a x, y axis lines through the origin
    plt.axhline(0, color="gray", linestyle="-")
    plt.axvline(0, color="gray", linestyle="-")
    # Add a title (specify font parameters with fontdict)
    plt.title(title, fontdict={"fontname": "Lucida Sans", "fontsize": 24})
    # X and Y labels
    plt.xlabel("X Axis")
    plt.ylabel("Y Axis")
    # specify X axis Tickmarks (will resize graph)
    plt.xticks(np.arange(-1, 6))
    # Set y-axis tick marks to be every 1
    plt.yticks(range(y_min, y_max+1))
    # Place legend outside plot at top right, making extra room for it
    plt.legend(title="Lines", loc='center left', bbox_to_anchor=(1.05, 0.5))
    # Get the directory of the current file
    currfile_dir = Path(__file__).parent
    # Replace spaces in title with underscores to create filename for saving figure
    filename = title.replace(" ", "_")
    # build the image file path
    filepath = currfile_dir / (f"{filename}.png")
    # Save figure (dpi 300 is good when saving so graph has high resolution)
    plt.savefig(filepath, dpi=600)
    # Show plot
    plt.show()


def main():
    plot_line_graph("Straight line and parabola", ["2 * x + 1", "(x-1)**2"], ["y = 2x + 1", r"y = (x-1)$^2$"])


if __name__ == '__main__':
    main()