Multiple plots (hold on) in MATLAB - Step-by-Step Execution

Concept Flow - Multiple plots (hold on)

Start

↓

Plot first graph

↓

hold on activated

↓

Plot second graph

↓

Plot more graphs if needed

↓

hold off to stop adding

↓

End

The flow shows plotting the first graph, turning 'hold on' to keep the plot, adding more plots, then turning 'hold off' to finish.

Execution Sample

MATLAB

x = 0:0.1:2*pi;
y1 = sin(x);
y2 = cos(x);
plot(x,y1)
hold on
plot(x,y2)
hold off

This code plots sine and cosine on the same graph using 'hold on' to keep the first plot visible.

Execution Table

Step	Action	Plot State	Output
1	Create x, y1, y2 arrays	No plot yet	Variables ready
2	plot(x,y1)	Plot shows sine curve	Sine curve visible
3	hold on	Hold activated	Next plots added on same graph
4	plot(x,y2)	Sine and cosine curves visible	Cosine curve added
5	hold off	Hold deactivated	No more plots added automatically
6	End	Final plot with sine and cosine	Plot window shows both curves

💡 All plots added; hold off stops further additions.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 4	Final
x	undefined	[0 0.1 ... 6.28]	[0 0.1 ... 6.28]	[0 0.1 ... 6.28]	[0 0.1 ... 6.28]
y1	undefined	sin(x) values	sin(x) values	sin(x) values	sin(x) values
y2	undefined	cos(x) values	cos(x) values	cos(x) values	cos(x) values

Key Moments - 2 Insights

Why does the second plot replace the first if 'hold on' is not used?

Without 'hold on', each plot command clears the previous plot, so only the last plot shows. See step 2 and 4 in execution_table where hold on is activated to keep both.

What happens if 'hold off' is not called after plotting multiple graphs?

'hold off' stops adding new plots on the same figure. If omitted, new plots will continue to add until MATLAB session ends or figure is closed. See step 5 for hold off effect.

Visual Quiz - 3 Questions

Look at the execution_table, what is the plot state after step 3?

A Only sine curve is visible; hold is off

B Hold is activated; next plots add on same graph

C Plot window is cleared

D Cosine curve is visible alone

💡 Hint

Check the 'Plot State' column at step 3 in execution_table

At which step does the cosine curve first appear on the plot?

A Step 4

B Step 3

C Step 2

D Step 5

💡 Hint

Look at the 'Output' column in execution_table for when cosine curve is added

If 'hold off' was not called, what would happen when plotting a third graph?

A The third graph would replace the previous two

B An error would occur

C The third graph would be added on the same plot

D The plot window would close

💡 Hint

Refer to key_moments about hold off effect and execution_table step 5

Concept Snapshot

plot(x,y) draws a graph.
hold on keeps current plot.
plot more graphs to add on same figure.
hold off stops adding plots.
Use to compare multiple data sets visually.

Full Transcript

This example shows how to plot multiple graphs on the same figure in MATLAB. First, we create x values and two y arrays for sine and cosine. We plot the sine curve. Then, we use 'hold on' to keep this plot visible. Next, we plot the cosine curve, which appears on the same graph. Finally, 'hold off' stops further plots from adding automatically. Without 'hold on', each new plot replaces the previous one. This technique helps compare multiple data sets visually in one figure.

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