10.3: Creating Simple Plots

The most common plotting function in MATLAB is plot. We can use it to plot a single point, a vector of values, or one vector against another vector.

Plotting a Single Point

To plot a single point, we can pass the x-coordinate, the y-coordinate, and an optional style specification. For example, the following command plots the point (2, 3) using a red star:

Example \(\PageIndex{1}\)

plot(2, 3, 'r*');

Solution

The first value is the x-coordinate, the second value is the y-coordinate, and the third input tells MATLAB how to draw the point. In this case, r means red and * means star marker.

 

 

 

Plotting One Vector

If you pass one vector to plot, MATLAB uses the vector values as the y-values. The x-values are automatically set to 1, 2, 3, and so on.

Example \(\PageIndex{2}\)

y = [2 4 9 10 12];

plot(y);

Solution

This creates a plot with five points. The y-values are 2, 4, 9, 10, and 12. The x-values are 1 through 5 because there are five elements in the vector.

 

Plotting y Versus x

Most of the time, we want to control both the x-values and the y-values. In that case, we pass two vectors to plot:

Example \(\PageIndex{3}\)

x = [2 4 6 8 10];

y = [2 4 9 10 12];

plot(x, y);

Solution

Here MATLAB plots each pair of values: (2, 2), (4, 4), (6, 9), (8, 10), and (10, 12).

By default, MATLAB connects the points with a thin solid blue line.

 

Caution

The x vector and y vector must have the same number of elements. If x has 5 values and y has 6 values, MATLAB will not know how to pair the data points correctly.

 

Customizing Plot Appearance

A default MATLAB plot is useful, but it is often not enough for a report or presentation. We usually want to adjust the color, marker, and line style so the plot is easier to read.

Category	Examples	Meaning
Colors	'b', 'r', 'g', 'k', 'm', 'c', 'y'	blue, red, green, black, magenta, cyan, yellow
Markers	'o', '*', '+', 'x', 's', 'd', '^'	circle, star, plus, x-mark, square, diamond, triangle
Line styles	'-', '--', ':', '-.'	solid, dashed, dotted, dash-dot

 

These symbols can be combined in one style string. For example:

plot(x, y, 'ro')     % red circle markers

plot(x, y, 'k--')    % black dashed line

plot(x, y, 'm*')     % magenta star markers

plot(x, y, 'gd-')    % green diamonds connected with a solid line

 

Example \(\PageIndex{4}\)

Projectile Motion

Let us use plotting to visualize a basic engineering problem: the trajectory of a projectile. Suppose a ball is launched at an angle with an initial velocity. If we ignore air resistance, the horizontal and vertical positions can be modeled as functions of time.

 

% Projectile trajectory example

clear; clc; clf;

angle = 45;          % launch angle in degrees
velocity = 35.0;     % initial velocity in m/s
g = 9.8;             % gravitational acceleration in m/s^2


timeSteps = 0:0.1:5;


xPos = velocity * cosd(angle) * timeSteps;
yPos = velocity * sind(angle) * timeSteps - 0.5 * g * timeSteps.^2;


plot(xPos, yPos, 'm*')
xlabel('Horizontal Position (m)')
ylabel('Vertical Position (m)')
title('Projectile Motion')
grid on

In the above example, notice the use of cosd and sind. These functions use angles measured in degrees. If the angle were in radians, we would use cos and sin instead. Also notice the dot in timeSteps.^2. This tells MATLAB to square each element of the time vector individually.

Solution

 

 

Helpful Plot Functions

MATLAB provides several functions that help us manage figure windows and make plots more descriptive.

Function	Purpose
clf	Clears the current figure window.
figure	Creates a new figure window or makes a specific figure active.
hold on	Keeps the current plot so that new plots are added to the same axes.
hold off	Turns off hold so new plots replace the old plot.
legend	Adds a legend that identifies plotted curves.
grid on	Adds grid lines to the plot.
xlabel	Adds a label below the x-axis.
ylabel	Adds a label next to the y-axis.
title	Adds a title above the plot.

 

Good habit

Every plot that you turn in for a lab report, homework assignment, or project should usually have a title, x-axis label, y-axis label, and, when needed, a legend.

 

Plotting Multiple Curves on One Figure

To plot more than one curve on the same axes, use hold on after the first plot. Then create the additional plots. When you are finished, use hold off.

 

Example \(\PageIndex{5}\)

Plotting four curves on the same figure.

clear; clc; clf;

x = 1:5;
y1 = [2 5 6 8 11];
y2 = [4 3 5 7 8];
y3 = [1 6 9 12 9];
y4 = [4 4 4 4 4];


figure(1)
plot(x, y1, 'k')
hold on
plot(x, y2, 'ko')
plot(x, y3, 'gd')
plot(x, y4, 'r--')
hold off


grid on
legend('y1', 'y2', 'y3', 'y4')
xlabel('x values')
ylabel('y values')
title('Four Curves on One Plot')

Solution

The order of labels in the legend matters. MATLAB matches the first legend label to the first plot, the second label to the second plot, and so on.

Exercise \(\PageIndex{1}\)

In MATLAB plot sin(x) and cos(x) on the same figure. Use x values ranging between 0 and 2\(\pi\). Add a title, legend, x-axis label, and y-axis label to the figure.