plot() function basics in MATLAB

Introduction

The plot() function helps you draw simple graphs to see how data changes. It makes numbers easy to understand by showing them as lines or points.

You want to see how sales change over months.

You need to compare temperatures on different days.

You want to check if a math formula works by drawing its curve.

You want to show how a student's scores improve over time.

Syntax

MATLAB

plot(x, y)
plot(x, y, 'style')
plot(y)
plot(x1, y1, x2, y2, ...)

x and y are lists or arrays of numbers of the same length.

The 'style' is a string that changes line color, marker, or line type.

Examples

Draws a line graph with points from x=1 to 5 and y values doubling x.

MATLAB

x = 1:5;
y = [2, 4, 6, 8, 10];
plot(x, y)

Plots y values against their index (1 to 5) automatically.

MATLAB

y = [1, 3, 5, 7, 9];
plot(y)

Plots a red dashed line showing y = x squared.

MATLAB

x = 0:0.1:1;
y = x.^2;
plot(x, y, 'r--')

Draws two lines on the same graph for comparison.

MATLAB

x1 = 1:3;
y1 = [2, 4, 6];
x2 = 1:3;
y2 = [3, 6, 9];
plot(x1, y1, x2, y2)

Sample Program

This program draws a blue line with circle markers showing y = 2x + 1. It adds a title and labels for clarity.

MATLAB

x = 1:10;
y = 2 * x + 1;
plot(x, y, 'b-o')
title('Simple Line Plot')
xlabel('X values')
ylabel('Y values')

Important Notes

If you only give one list, plot() uses the index numbers as x values.

You can add labels and titles to make your graph easier to understand.

Use different styles like 'r--' for red dashed lines or 'g*' for green stars.

Summary

plot() draws graphs to help you see data trends.

You can plot one or more sets of points with different styles.

Adding titles and labels makes your graph clear and useful.

Practice

1. What does the plot() function do in MATLAB?

easy

A. It calculates the sum of two numbers.

B. It clears the command window.

C. It saves data to a file.

D. It creates a graph to visually display data points.

Solution

1. Step 1: Understand the purpose of plot()

   The plot() function is used to draw graphs in MATLAB, which helps visualize data points.

2. Step 2: Compare with other options

   Options A, C, and D describe unrelated functions like summing numbers, saving files, or clearing the window, which plot() does not do.

3. Final Answer:

   It creates a graph to visually display data points. -> Option D

4. Quick Check:

   plot() = graph display [OK]

Hint: Remember: plot() always draws graphs [OK]

Common Mistakes:

• Confusing plot() with data calculation functions
• Thinking plot() saves files
• Assuming plot() clears the screen

2. Which of the following is the correct syntax to plot y versus x in MATLAB?

easy

A. plot(x:y)

B. plot(y, x)

C. plot(x, y)

D. plot(x + y)

Solution

1. Step 1: Recall the syntax of plot()

   The correct syntax to plot y versus x is plot(x, y), where x and y are vectors of the same length.

2. Step 2: Check other options

   plot(y, x) reverses the order, plotting x versus y, which is incorrect. plot(x:y) uses a colon operator incorrectly. plot(x + y) tries to plot the sum, which is invalid.

3. Final Answer:

   plot(x, y) -> Option C

4. Quick Check:

   plot(x, y) = correct syntax [OK]

Hint: Always put x first, then y in plot() [OK]

Common Mistakes:

• Swapping x and y arguments
• Using colon operator inside plot()
• Trying to plot sums instead of vectors

3. What will be the output of the following MATLAB code?

x = 1:3;
y = [2 4 6];
plot(x, y)

medium

A. A plot with points (1,2), (2,4), (3,6) connected by lines.

B. A plot with points (2,1), (4,2), (6,3) connected by lines.

C. An error because x and y have different lengths.

D. A plot with points (1,1), (2,2), (3,3) connected by lines.

Solution

1. Step 1: Analyze vectors x and y

   x is [1 2 3] and y is [2 4 6], both have length 3, so they can be plotted against each other.

2. Step 2: Understand plot output

   The plot will connect points (1,2), (2,4), and (3,6) with lines.

3. Final Answer:

   A plot with points (1,2), (2,4), (3,6) connected by lines. -> Option A

4. Quick Check:

   plot(x, y) = points (1,2), (2,4), (3,6) [OK]

Hint: Match x and y points by position in vectors [OK]

Common Mistakes:

• Mixing up x and y coordinates
• Assuming error due to vector length mismatch
• Confusing point coordinates

4. Identify the error in this MATLAB code:

x = 1:5;
y = [2 4 6];
plot(x, y)

medium

A. Vectors x and y have different lengths causing an error.

B. plot() function is misspelled.

C. x and y must be column vectors, not row vectors.

D. No error; the plot will display correctly.

Solution

1. Step 1: Check lengths of x and y

   x has length 5, y has length 3. plot() requires vectors of equal length.

2. Step 2: Understand consequence

   Because lengths differ, MATLAB will produce an error when trying to plot.

3. Final Answer:

   Vectors x and y have different lengths causing an error. -> Option A

4. Quick Check:

   Equal length vectors needed [OK]

Hint: Ensure x and y vectors have same length before plotting [OK]

Common Mistakes:

• Ignoring vector length mismatch
• Thinking row vs column vector matters here
• Assuming plot() spelling error

5. You want to plot the function y = x^2 for x from -3 to 3 with a red dashed line. Which code correctly does this?

hard

A. x = -3:0.1:3; y = x.^2; plot(x, y, 'blue-')

B. x = -3:0.1:3; y = x.^2; plot(x, y, 'r--')

C. x = -3:0.1:3; y = x^2; plot(y, x, 'r--')

D. x = -3:3; y = x^2; plot(x, y, 'red-dash')

Solution

1. Step 1: Define x and y correctly

   x must be a vector from -3 to 3 with small steps (e.g., 0.1). y = x squared element-wise uses .^2.

2. Step 2: Use correct plot style

   The style string 'r--' means red dashed line. x = -3:0.1:3; y = x.^2; plot(x, y, 'r--') uses this correctly.

3. Step 3: Check other options

   x = -3:3; y = x^2; plot(x, y, 'red-dash') uses invalid syntax for y and style. x = -3:0.1:3; y = x^2; plot(y, x, 'r--') swaps x and y. x = -3:0.1:3; y = x.^2; plot(x, y, 'blue-') uses blue solid line, not red dashed.

4. Final Answer:

   x = -3:0.1:3; y = x.^2; plot(x, y, 'r--') -> Option B

5. Quick Check:

   Element-wise power + 'r--' = red dashed line [OK]

Hint: Use .^ for element-wise power and 'r--' for red dashed line [OK]

Common Mistakes:

• Using ^ instead of .^ for vectors
• Swapping x and y in plot()
• Wrong line style string