Scatter plots in MATLAB - Mini Project: Build & Apply

Scatter plots

📖 Scenario: You are a scientist who collected data about the heights and weights of a group of people. You want to visualize this data to see if there is any pattern between height and weight.

🎯 Goal: Create a scatter plot in MATLAB that shows the relationship between height and weight.

📋 What You'll Learn

Create two vectors called height and weight with given data

Create a variable called markerSize to control the size of the scatter plot points

Use the scatter function with height, weight, and markerSize

Add labels to the x-axis and y-axis

Display the scatter plot

💡 Why This Matters

🌍 Real World

Scientists and data analysts often use scatter plots to see if two things are related, like height and weight.

💼 Career

Knowing how to create scatter plots is useful in jobs involving data visualization, research, and reporting.

1

Create the data vectors

Create a vector called height with these exact values: [150, 160, 170, 180, 190] and a vector called weight with these exact values: [50, 60, 70, 80, 90].

MATLAB

% Create vectors height and weight with given values
% Your code here

Hint

Use square brackets [] to create vectors in MATLAB.

2

Set the marker size

Create a variable called markerSize and set it to 100 to control the size of the scatter plot points.

MATLAB

height = [150, 160, 170, 180, 190];
weight = [50, 60, 70, 80, 90];
% Create markerSize variable and set it to 100
% Your code here

Hint

Just assign the number 100 to the variable markerSize.

3

Create the scatter plot

Use the scatter function with height, weight, and markerSize to create the scatter plot.

MATLAB

height = [150, 160, 170, 180, 190];
weight = [50, 60, 70, 80, 90];
markerSize = 100;
% Create scatter plot using scatter function
% Your code here

Hint

Use scatter(height, weight, markerSize) to plot the points.

4

Add labels and display the plot

Add the x-axis label 'Height (cm)' using xlabel and the y-axis label 'Weight (kg)' using ylabel. Then display the plot.

MATLAB

height = [150, 160, 170, 180, 190];
weight = [50, 60, 70, 80, 90];
markerSize = 100;
scatter(height, weight, markerSize);
% Add x-axis and y-axis labels
% Your code here

Hint

Use xlabel('Height (cm)') and ylabel('Weight (kg)') to add labels.

Practice

1. What does a scatter plot in MATLAB primarily show?

easy

A. A bar chart of grouped data

B. A continuous line connecting data points

C. Points representing pairs of values on an X-Y graph

D. A pie chart of category percentages

Solution

1. Step 1: Understand scatter plot purpose

   A scatter plot shows individual points for pairs of values, usually X and Y coordinates.

2. Step 2: Compare options to definition

   Only Points representing pairs of values on an X-Y graph describes points on an X-Y graph, which matches scatter plot behavior.

3. Final Answer:

   Points representing pairs of values on an X-Y graph -> Option C

4. Quick Check:

   Scatter plot = points on X-Y graph [OK]

Hint: Scatter plots show points, not lines or bars [OK]

Common Mistakes:

• Confusing scatter plot with line plot
• Thinking scatter plots show bars or pie slices
• Assuming scatter plots connect points

2. Which of the following is the correct MATLAB syntax to create a scatter plot of vectors x and y?

easy

A. plot.scatter(x, y)

B. scatter(x, y)

C. scatterplot(x, y)

D. scatter x, y

Solution

1. Step 1: Recall MATLAB scatter syntax

   The correct function to create scatter plots is scatter(x, y).

2. Step 2: Check other options for syntax errors

   Options A, C, and D are not valid MATLAB commands for scatter plots.

3. Final Answer:

   scatter(x, y) -> Option B

4. Quick Check:

   scatter function syntax = scatter(x, y) [OK]

Hint: Use scatter() function with parentheses and commas [OK]

Common Mistakes:

• Using dot notation like plot.scatter
• Typing scatterplot instead of scatter
• Omitting parentheses or commas

3. What will the following MATLAB code display?

x = 1:5;
y = [2 4 6 8 10];
scatter(x, y, 100, 'r', 'filled')

medium

A. A scatter plot with red filled circles sized 100 at points (1,2), (2,4), ..., (5,10)

B. A line plot connecting points from (1,2) to (5,10)

C. An error because 'filled' is not a valid option

D. A scatter plot with default size and blue color points

Solution

1. Step 1: Understand scatter parameters

   The code uses scatter(x, y, 100, 'r', 'filled') which sets point size to 100, color red ('r'), and fills the markers.

2. Step 2: Interpret the plot output

   The plot shows red filled circles at points (1,2), (2,4), (3,6), (4,8), and (5,10).

3. Final Answer:

   A scatter plot with red filled circles sized 100 at points (1,2), (2,4), ..., (5,10) -> Option A

4. Quick Check:

   scatter(x,y,100,'r','filled') = red filled circles [OK]

Hint: Size, color, and fill control marker appearance [OK]

Common Mistakes:

• Thinking 'filled' causes error
• Confusing scatter with line plot
• Ignoring size and color parameters

4. Identify the error in this MATLAB code for a scatter plot:

x = 1:4;
y = [5 6 7];
scatter(x, y)

medium

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

B. scatter requires a third argument for size

C. scatter cannot plot numeric vectors

D. No error; code runs fine

Solution

1. Step 1: Check vector lengths

   Vector x has length 4, but y has length 3, which is invalid for scatter.

2. Step 2: Understand scatter input requirements

   Scatter requires x and y to be the same length to plot pairs of points.

3. Final Answer:

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

4. Quick Check:

   Length mismatch in x,y vectors = error [OK]

Hint: x and y must be same length for scatter [OK]

Common Mistakes:

• Assuming scatter auto-fixes length mismatch
• Thinking size argument is mandatory
• Believing scatter can't plot numeric data

5. You have two groups of data points:

x1 = [1 2 3]; y1 = [4 5 6];
x2 = [1 2 3]; y2 = [7 8 9];

How can you plot both groups on the same scatter plot with different colors using filled markers and add a legend?

hard

A. plot(x1, y1, 'b'); plot(x2, y2, 'r'); legend('Group 1', 'Group 2')

B. scatter(x1, y1, 'b', 'filled'); scatter(x2, y2, 'r', 'filled'); legend('Group 1', 'Group 2')

C. scatter(x1, y1, 'b'); hold on; scatter(x2, y2, 'r'); legend('Group 1', 'Group 2')

D. scatter(x1, y1, 'b', 'filled'); hold on; scatter(x2, y2, 'r', 'filled'); legend('Group 1', 'Group 2')

Solution

1. Step 1: Plot first group with filled blue markers

   Use scatter(x1, y1, 'b', 'filled') to plot blue filled points.

2. Step 2: Use hold on before plotting second group

   To overlay plots, hold on keeps the first plot visible before adding the second.

3. Step 3: Plot second group with filled red markers and add legend

   Plot second group with scatter(x2, y2, 'r', 'filled') and add legend for clarity.

4. Final Answer:

   scatter(x1, y1, 'b', 'filled'); hold on; scatter(x2, y2, 'r', 'filled'); legend('Group 1', 'Group 2') -> Option D

5. Quick Check:

   Use hold on to overlay scatter plots with legend [OK]

Hint: Use hold on to combine scatter plots with different colors [OK]

Common Mistakes:

• Forgetting hold on causes second plot to overwrite first
• Using plot instead of scatter for points
• Omitting legend or using incorrect syntax