Multiple plots (hold on) in MATLAB - Mini Project: Build & Apply

Multiple plots using hold on in MATLAB

📖 Scenario: You are working on a simple data visualization project where you want to compare two sets of data on the same graph. This is common when you want to see how two trends relate to each other over the same range.

🎯 Goal: Build a MATLAB script that plots two different lines on the same graph using hold on. This will help you visualize both data sets together clearly.

📋 What You'll Learn

Create two vectors of data points

Plot the first data set using the plot function

Use hold on to keep the first plot visible

Plot the second data set on the same graph

Add labels for the x-axis and y-axis

Add a legend to distinguish the two plots

💡 Why This Matters

🌍 Real World

Plotting multiple data sets on the same graph is useful in science, engineering, and business to compare trends and relationships clearly.

💼 Career

Data visualization skills like plotting multiple lines help in roles such as data analyst, engineer, and researcher to communicate insights effectively.

1

Create the data vectors

Create two vectors called x and y1. Set x to the values [1 2 3 4 5] and y1 to the values [2 4 6 8 10].

MATLAB

% Create vectors x and y1 with the specified values
% Your code here

Hint

Use square brackets to create vectors in MATLAB, like x = [1 2 3].

2

Add the second data vector

Create a second vector called y2 with the values [1 3 5 7 9].

MATLAB

x = [1 2 3 4 5];
y1 = [2 4 6 8 10];
% Create vector y2 with the specified values
% Your code here

Hint

Remember to use square brackets to create the vector.

3

Plot the first data set and hold the plot

Use plot(x, y1) to plot the first data set. Then use hold on to keep the plot active for adding more data.

MATLAB

x = [1 2 3 4 5];
y1 = [2 4 6 8 10];
y2 = [1 3 5 7 9];
% Plot y1 vs x
% Use hold on to keep the plot
% Your code here

Hint

Use plot(x, y1) to draw the first line and hold on to keep the figure ready for more plots.

4

Plot the second data set and add labels and legend

Plot the second data set using plot(x, y2). Then add x-axis label 'X values', y-axis label 'Y values', and a legend with labels 'Data 1' and 'Data 2'. Finally, use hold off to finish plotting.

MATLAB

x = [1 2 3 4 5];
y1 = [2 4 6 8 10];
y2 = [1 3 5 7 9];
plot(x, y1);
hold on;
% Plot y2 vs x
% Add labels and legend
% Use hold off
% Your code here

Hint

Use xlabel, ylabel, and legend functions to add descriptions. Use hold off to stop adding plots.

Practice

1. What does the hold on command do in MATLAB plotting?

easy

A. Clears the current figure before plotting new data.

B. Allows multiple plots to be drawn on the same figure without erasing previous plots.

C. Saves the current plot to a file.

D. Changes the color of the plot lines automatically.

Solution

1. Step 1: Understand the purpose of hold on

   The hold on command tells MATLAB to keep the current plot and add new plots on top of it.

2. Step 2: Compare with other options

   Other commands like hold off or default plotting clear the figure before new plots, so they do not allow multiple plots together.

3. Final Answer:

   Allows multiple plots to be drawn on the same figure without erasing previous plots. -> Option B

4. Quick Check:

   hold on = multiple plots on one figure [OK]

Hint: Remember: hold on means keep adding plots [OK]

Common Mistakes:

• Thinking hold on clears the figure
• Confusing hold on with saving plots
• Assuming hold on changes plot colors

2. Which of the following is the correct way to plot two lines on the same figure in MATLAB?

easy

A. plot(x1, y1); hold on; plot(x2, y2); hold off;

B. hold on; plot(x1, y1); hold off; plot(x2, y2);

C. plot(x1, y1); plot(x2, y2);

D. plot(x1, y1); hold off; plot(x2, y2);

Solution

1. Step 1: Identify the correct sequence to keep multiple plots

   Using hold on after the first plot keeps it visible for the second plot.

2. Step 2: Confirm proper use of hold off

   Calling hold off after plotting both lines resets plotting behavior for future plots.

3. Final Answer:

   plot(x1, y1); hold on; plot(x2, y2); hold off; -> Option A

4. Quick Check:

   Use hold on between plots to combine them [OK]

Hint: Use hold on right after first plot to add more [OK]

Common Mistakes:

• Not using hold on before second plot
• Using hold off too early
• Plotting without hold on clears previous plot

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

t = 0:0.1:1;
plot(t, t);
hold on;
plot(t, t.^2);

medium

A. Two plots will appear: a straight line and a curve on the same figure.

B. Only the plot of t vs t^2 will be shown.

C. An error will occur because hold on is used incorrectly.

D. Only the plot of t vs t will be shown.

Solution

1. Step 1: Analyze the first plot command

   The first plot draws a straight line y = t on the figure.

2. Step 2: Understand the effect of hold on and second plot

   Using hold on keeps the first plot visible, so the second plot y = t^2 is drawn on the same figure as a curve.

3. Final Answer:

   Two plots will appear: a straight line and a curve on the same figure. -> Option A

4. Quick Check:

   hold on keeps previous plot, so both appear [OK]

Hint: hold on keeps old plots visible for new ones [OK]

Common Mistakes:

• Thinking only last plot shows
• Assuming hold on causes error
• Confusing plot commands order

4. Identify the error in this MATLAB code that tries to plot two lines on the same figure:

plot(x, y1)
hold off
plot(x, y2)

medium

A. There is no error; the code works correctly.

B. The variable x is not defined.

C. hold off should be called before the first plot.

D. hold off should be replaced with hold on to keep both plots visible.

Solution

1. Step 1: Understand the effect of hold off

   Calling hold off resets plotting to clear the figure before the next plot.

2. Step 2: Identify correct command to keep plots

   To keep both plots visible, hold on should be used instead of hold off.

3. Final Answer:

   hold off should be replaced with hold on to keep both plots visible. -> Option D

4. Quick Check:

   Use hold on to combine plots, not hold off [OK]

Hint: hold off clears plots; use hold on to keep them [OK]

Common Mistakes:

• Using hold off instead of hold on
• Thinking hold off adds plots
• Ignoring hold commands altogether

5. You want to plot three different data sets on the same figure with different colors and then reset plotting to default for future plots. Which sequence of commands is correct?

hard

A. plot(x1, y1, 'r'); plot(x2, y2, 'g'); plot(x3, y3, 'b'); hold on; hold off;

B. hold on; plot(x1, y1, 'r'); plot(x2, y2, 'g'); plot(x3, y3, 'b');

C. plot(x1, y1, 'r'); hold on; plot(x2, y2, 'g'); plot(x3, y3, 'b'); hold off;

D. plot(x1, y1, 'r'); hold off; plot(x2, y2, 'g'); hold on; plot(x3, y3, 'b');

Solution

1. Step 1: Use hold on after first plot to combine multiple plots

   After plotting the first data set, hold on keeps the figure active for additional plots.

2. Step 2: Plot all data sets with specified colors and then reset with hold off

   Plotting the second and third data sets adds them to the same figure. Calling hold off resets plotting behavior for future plots.

3. Final Answer:

   plot(x1, y1, 'r'); hold on; plot(x2, y2, 'g'); plot(x3, y3, 'b'); hold off; -> Option C

4. Quick Check:

   hold on after first plot, hold off after last plot [OK]

Hint: hold on after first plot, hold off after last [OK]

Common Mistakes:

• Not calling hold off after multiple plots
• Calling hold on before first plot
• Mixing hold on and hold off incorrectly