Time series with fill_between in Matplotlib - Step-by-Step Execution

Concept Flow - Time series with fill_between

Prepare time series data

↓

Plot line graph of data

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Use fill_between to shade area

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Show plot with shaded area

We first prepare time series data, then plot it as a line graph, next use fill_between to shade the area under or between lines, and finally display the plot.

Execution Sample

Matplotlib

import matplotlib.pyplot as plt
import numpy as np
x = np.arange(10)
y = np.sin(x / 2)
plt.plot(x, y)
plt.fill_between(x, y, color='skyblue', alpha=0.5)
plt.show()

This code plots a sine wave over 10 points and shades the area under the curve with a light blue color.

Execution Table

Step	Action	Variable/Function	Result/Effect
1	Import matplotlib.pyplot and numpy	plt, np	Modules ready for plotting and data
2	Create x as array 0 to 9	x = np.arange(10)	x = [0 1 2 3 4 5 6 7 8 9]
3	Calculate y as sine of x/2	y = np.sin(x / 2)	y approx [0.0, 0.479, 0.841, 0.909, 0.997, 0.598, 0.141, -0.351, -0.757, -0.978]
4	Plot line graph of y vs x	plt.plot(x, y)	Line graph drawn on plot
5	Fill area under y with color skyblue and alpha 0.5	plt.fill_between(x, y, color='skyblue', alpha=0.5)	Area under line shaded light blue
6	Show the plot	plt.show()	Plot window opens with line and shaded area

💡 Plot displayed, execution ends

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
x	undefined	[0 1 2 3 4 5 6 7 8 9]	[0 1 2 3 4 5 6 7 8 9]	[0 1 2 3 4 5 6 7 8 9]
y	undefined	undefined	[0.0 0.47942554 0.84147098 0.90929743 0.99749499 0.59847214 0.14112001 -0.35078323 -0.7568025 -0.97753012]	[0.0 0.47942554 0.84147098 0.90929743 0.99749499 0.59847214 0.14112001 -0.35078323 -0.7568025 -0.97753012]

Key Moments - 3 Insights

Why does fill_between shade the area under the curve and not above?

fill_between shades the area between the x-axis (default y=0) and the y values. Since y is positive or negative, the shaded area follows the curve down to zero, as shown in step 5 of the execution_table.

What happens if y has negative values in fill_between?

fill_between shades between y and zero, so if y is negative, the shaded area appears below the x-axis line, visible in the negative y values in step 3 and shaded in step 5.

Can fill_between shade between two different lines?

Yes, by providing two y arrays to fill_between, it shades the area between them. Here, only one y is given, so it shades between y and zero (step 5).

Visual Quiz - 3 Questions

Look at the execution_table at step 3, what is the approximate value of y at x=3?

A 0.76

B 0.24

C 0.99

D -0.28

💡 Hint

Check the y array values in step 3 of execution_table; y at index 3 is about 0.909

At which step does the shaded area get added to the plot?

A Step 4

B Step 5

C Step 6

D Step 3

💡 Hint

Look at the Action column in execution_table; fill_between is called at step 5

If we change fill_between to fill between y and y+1, how would the shaded area change?

A It would shade area between y and y+1, above the curve

B It would shade below y only

C No shading would appear

D It would shade between 0 and y only

💡 Hint

Recall fill_between can take two y arrays to shade between, as explained in key_moments point 3

Concept Snapshot

Time series with fill_between:
- Prepare x (time) and y (values) arrays
- Plot line graph with plt.plot(x, y)
- Use plt.fill_between(x, y, color, alpha) to shade area under curve
- Shows visually the area between curve and baseline (default y=0)
- Useful for highlighting ranges or confidence intervals

Full Transcript

This visual execution traces how to plot a time series line graph and shade the area under it using matplotlib's fill_between. First, we import necessary modules. Then, we create x as a sequence of numbers and y as sine values of x divided by 2. We plot y against x as a line. Next, fill_between shades the area between the curve and the x-axis with a light blue color. Finally, the plot is displayed. Key points include understanding that fill_between shades between two y-values (default zero and y), so negative y values shade below the axis. We also see how variables x and y change step-by-step. The quiz checks understanding of y values, when shading happens, and how changing fill_between parameters affects shading.