Markers on data points in Matplotlib - Mini Project: Build & Apply

Markers on data points

📖 Scenario: You are analyzing sales data for a small store. You want to visualize the sales numbers for each day of the week using a simple line chart. To make the data points clear, you will add markers on each point.

🎯 Goal: Create a line plot of sales data with markers on each data point to clearly show the sales values for each day.

📋 What You'll Learn

Create a list called days with the days of the week as strings.

Create a list called sales with the sales numbers for each day.

Use matplotlib.pyplot to plot the sales data.

Add markers on each data point using the marker parameter.

Display the plot with plt.show().

💡 Why This Matters

🌍 Real World

Markers on data points help highlight exact values in charts, making it easier to understand trends and compare data in reports or presentations.

💼 Career

Data analysts and scientists use markers in plots to communicate insights clearly to stakeholders and to explore data visually during analysis.

1

Create the sales data lists

Create a list called days with these exact values: 'Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat', 'Sun'. Also create a list called sales with these exact values: 150, 200, 170, 220, 180, 190, 210.

Matplotlib

# Create the days and sales lists
# Your code here

Hint

Use square brackets to create lists. Separate items with commas.

2

Import matplotlib and set up the plot

Import matplotlib.pyplot as plt. This will allow you to create the plot.

Matplotlib

days = ['Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat', 'Sun']
sales = [150, 200, 170, 220, 180, 190, 210]

# Import matplotlib.pyplot as plt
# Your code here

Hint

Use import matplotlib.pyplot as plt to import the plotting library.

3

Plot the sales data with markers

Use plt.plot() to plot days on the x-axis and sales on the y-axis. Add markers on each data point by setting the marker parameter to 'o'.

Matplotlib

import matplotlib.pyplot as plt

days = ['Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat', 'Sun']
sales = [150, 200, 170, 220, 180, 190, 210]

# Plot days vs sales with circle markers
# Your code here

Hint

Use marker='o' inside plt.plot() to add circle markers.

4

Display the plot

Use plt.show() to display the plot with markers.

Matplotlib

import matplotlib.pyplot as plt

days = ['Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat', 'Sun']
sales = [150, 200, 170, 220, 180, 190, 210]

plt.plot(days, sales, marker='o')

# Show the plot
# Your code here

Hint

Call plt.show() to open the plot window.

Practice

1. What does the marker parameter do in a matplotlib plot?

easy

A. It specifies the shape of the points shown on the plot.

B. It changes the color of the plot line.

C. It sets the thickness of the plot line.

D. It adds grid lines to the plot.

Solution

1. Step 1: Understand the role of the marker parameter

   The marker parameter controls how individual data points appear on the plot, such as circles, squares, or triangles.

2. Step 2: Differentiate marker from other parameters

   Other parameters like color or linewidth control line appearance, not point shapes.

3. Final Answer:

   It specifies the shape of the points shown on the plot. -> Option A

4. Quick Check:

   marker = point shape [OK]

Hint: Marker shapes show points clearly on plots [OK]

Common Mistakes:

• Confusing marker with color or line style
• Thinking marker adds grid lines
• Assuming marker changes line thickness

2. Which of the following is the correct way to plot data points with circle markers using matplotlib?

easy

A. plt.plot(x, y, marker='o')

B. plt.plot(x, y, marker='x')

C. plt.plot(x, y, marker='s')

D. plt.plot(x, y, marker='^')

Solution

1. Step 1: Recall marker symbols in matplotlib

   In matplotlib, 'o' is the symbol for circle markers.

2. Step 2: Match marker symbol to circle

   Other options: 'x' is cross, 's' is square, '^' is triangle up.

3. Final Answer:

   plt.plot(x, y, marker='o') -> Option A

4. Quick Check:

   Circle marker = 'o' [OK]

Hint: Circle marker is 'o' in matplotlib [OK]

Common Mistakes:

• Using 'x' or 's' for circle markers
• Forgetting to put marker in quotes
• Using marker parameter incorrectly

3. What will the following code display?

import matplotlib.pyplot as plt
x = [1, 2, 3]
y = [4, 5, 6]
plt.plot(x, y, marker='^')
plt.show()

medium

A. A line plot with cross markers at each data point.

B. A scatter plot with circle markers.

C. A bar chart with triangle markers.

D. A line plot with triangle-up markers at each data point.

Solution

1. Step 1: Understand the code plot type

   The code uses plt.plot with x and y lists, so it creates a line plot.

2. Step 2: Interpret the marker parameter

   Marker '^' means triangle-up shape at each data point on the line.

3. Final Answer:

   A line plot with triangle-up markers at each data point. -> Option D

4. Quick Check:

   marker='^' means triangle-up markers [OK]

Hint: Marker '^' means triangle-up on line plot [OK]

Common Mistakes:

• Confusing plt.plot with scatter plot
• Misreading marker '^' as cross or circle
• Thinking it creates a bar chart

4. Identify the error in this code snippet:

import matplotlib.pyplot as plt
x = [1, 2, 3]
y = [3, 2, 1]
plt.plot(x, y, marker=o)
plt.show()

medium

A. The x and y lists have different lengths.

B. The marker value should be a string, so marker='o'.

C. plt.plot cannot use markers.

D. plt.show() is missing parentheses.

Solution

1. Step 1: Check marker parameter syntax

   The marker value must be a string, so it should be written as marker='o', not marker=o.

2. Step 2: Verify other parts of the code

   x and y lists have same length; plt.plot supports markers; plt.show() is correctly called.

3. Final Answer:

   The marker value should be a string, so marker='o'. -> Option B

4. Quick Check:

   Marker must be a string in quotes [OK]

Hint: Put marker symbols inside quotes [OK]

Common Mistakes:

• Forgetting quotes around marker symbol
• Assuming plt.plot can't use markers
• Misreading plt.show syntax

5. You want to plot two lines on the same graph with different markers: circles for the first line and squares for the second. Which code correctly does this?

hard

A. plt.plot(x1, y1, marker='x')
plt.plot(x2, y2, marker='^')
plt.show()

B. plt.plot(x1, y1, marker='s')
plt.plot(x2, y2, marker='o')
plt.show()

C. plt.plot(x1, y1, marker='o')
plt.plot(x2, y2, marker='s')
plt.show()

D. plt.plot(x1, y1, marker='o', marker='s')
plt.plot(x2, y2)
plt.show()

Solution

1. Step 1: Assign markers to each line separately

   Each call to plt.plot can have its own marker parameter to set point shapes.

2. Step 2: Match markers to lines as required

   First line uses 'o' for circles, second line uses 's' for squares.

3. Step 3: Check incorrect options

   plt.plot(x1, y1, marker='s') plt.plot(x2, y2, marker='o') plt.show() swaps markers; C uses wrong markers; D tries two markers in one call, which is invalid.

4. Final Answer:

   plt.plot(x1, y1, marker='o') plt.plot(x2, y2, marker='s') plt.show() -> Option C

5. Quick Check:

   Separate plot calls, each with correct marker [OK]

Hint: Use separate plot calls with different markers [OK]

Common Mistakes:

• Trying to set two markers in one plot call
• Swapping marker symbols by mistake
• Not calling plt.show() after plotting