Cursor and event handling in Matplotlib - Mini Project: Build & Apply

Interactive Plot with Cursor and Event Handling

📖 Scenario: You are working with a simple line plot showing sales data over a week. You want to make the plot interactive so that when you move your mouse over the plot, a cursor line follows your mouse, and when you click on the plot, the exact data point coordinates are printed.

🎯 Goal: Create a matplotlib plot with a cursor that moves with the mouse and prints the coordinates of the clicked point.

📋 What You'll Learn

Create a line plot with given sales data.

Add a vertical cursor line that follows the mouse movement.

Print the x and y coordinates when the plot is clicked.

💡 Why This Matters

🌍 Real World

Interactive plots help analysts explore data visually and get exact values by moving the mouse or clicking points.

💼 Career

Data scientists and analysts often create interactive visualizations to communicate insights clearly and allow users to explore data.

1

Create the sales data and plot

Create a list called days with values [1, 2, 3, 4, 5, 6, 7] representing days of the week. Create a list called sales with values [10, 15, 7, 12, 20, 18, 25]. Then, import matplotlib.pyplot as plt and plot sales against days using plt.plot(days, sales). Finally, call plt.show() to display the plot.

Matplotlib

# Create days and sales lists
# Import matplotlib.pyplot as plt
# Plot sales vs days
# Show the plot
# Your code here

Need a hint?

Remember to import matplotlib.pyplot as plt before plotting.

2

Add a vertical cursor line

After plotting, create a variable called cursor_line that stores a vertical line on the plot using plt.axvline at position x=0 with color 'gray' and linestyle '--'. This line will act as the cursor.

Matplotlib

days = [1, 2, 3, 4, 5, 6, 7]
sales = [10, 15, 7, 12, 20, 18, 25]
import matplotlib.pyplot as plt
plt.plot(days, sales)
# Create a vertical line at x=0 with gray dashed style and assign it to cursor_line
# Your code here
plt.show()

Need a hint?

Use plt.axvline to add a vertical line at x=0.

3

Add mouse move event to update cursor

Define a function called on_mouse_move that takes an event parameter. Inside the function, check if event.xdata is not None. If so, update the cursor_line position by setting its xdata to [event.xdata, event.xdata] and then call plt.gcf().canvas.draw_idle() to refresh the plot. Connect this function to the figure's motion_notify_event using plt.gcf().canvas.mpl_connect.

Matplotlib

days = [1, 2, 3, 4, 5, 6, 7]
sales = [10, 15, 7, 12, 20, 18, 25]
import matplotlib.pyplot as plt
plt.plot(days, sales)
cursor_line = plt.axvline(x=0, color='gray', linestyle='--')
# Define on_mouse_move(event) to update cursor_line position
# Connect on_mouse_move to motion_notify_event
plt.show()

Need a hint?

Use set_xdata on the cursor line and redraw the canvas.

4

Add click event to print coordinates

Define a function called on_click that takes an event parameter. Inside the function, check if event.xdata and event.ydata are not None. If so, print the message Clicked at x={event.xdata:.2f}, y={event.ydata:.2f} using an f-string. Connect this function to the figure's button_press_event using plt.gcf().canvas.mpl_connect. Finally, call plt.show() to display the interactive plot.

Matplotlib

days = [1, 2, 3, 4, 5, 6, 7]
sales = [10, 15, 7, 12, 20, 18, 25]
import matplotlib.pyplot as plt
plt.plot(days, sales)
cursor_line = plt.axvline(x=0, color='gray', linestyle='--')

def on_mouse_move(event):
    if event.xdata is not None:
        cursor_line.set_xdata([event.xdata, event.xdata])
        plt.gcf().canvas.draw_idle()

plt.gcf().canvas.mpl_connect('motion_notify_event', on_mouse_move)
# Define on_click(event) to print clicked coordinates
# Connect on_click to button_press_event
plt.show()

Need a hint?

Use button_press_event to detect clicks and print coordinates.