Line Plot

When to use line plots

Line plots are best for:

• Trends over time
• Comparing changes across periods

A line plot is Matplotlib’s default: plt.plot(data) alone draws a line, connecting each point to the next in the order the data appears. Points aren’t interpolated by magic — Matplotlib just draws straight segments between consecutive (x, y) pairs, so ordering your data matters.

diagram Building a line plot mermaid

  flowchart LR
  A["x values"] --> C["plt.plot(x, y)"]
  B["y values"] --> C
  C --> D["marker + linestyle"]
  D --> E["title / xlabel / ylabel"]
  E --> F["legend (if multiple series)"]

How x/y arrays become a line, then a labelled, legible chart.

Basic line plot

Line plot

import matplotlib.pyplot as plt
 
x = [1, 2, 3, 4, 5]
y = [120, 140, 130, 160, 155]
 
plt.figure(figsize=(7, 4))
plt.plot(x, y, marker="o")
plt.title("Orders over time")
plt.xlabel("Day")
plt.ylabel("Orders")
plt.grid(True, alpha=0.3)
plt.tight_layout()
plt.show()

Multiple lines

Multiple lines

import matplotlib.pyplot as plt
 
x = [1, 2, 3, 4, 5]
y1 = [120, 140, 130, 160, 155]
y2 = [100, 110, 120, 125, 140]
 
plt.figure(figsize=(7, 4))
plt.plot(x, y1, label="Product A")
plt.plot(x, y2, label="Product B")
 
plt.title("Orders over time")
plt.xlabel("Day")
plt.ylabel("Orders")
plt.legend()
plt.grid(True, alpha=0.3)
plt.tight_layout()
plt.show()

Visualize it

Watch a line plot draw itself point by point — each new value connects to the previous one with a straight segment:

sketch A line plot connects points in order p5.js Pause Restart

Matplotlib draws a straight segment between each consecutive (x, y) pair as it walks through the data.

From the book

McKinney shows that ax.plot(data, linestyle="dashed", drawstyle="steps-post") changes how points connect, not just their color. By default Matplotlib linearly interpolates between points — drawstyle is handy when your data represents a step function (like a price that only changes at specific moments) rather than something continuous.

Tip

If lines cross and the plot becomes confusing, consider faceting (multiple subplots) or focusing on fewer series.

Next

Continue to Bar Chart and Horizontal Bar to compare categories instead of trends.

🧪 Try It Yourself

Exercise 1 – Plot with a marker

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Exercise 2 – Label a series for the legend

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Exercise 3 – Step drawstyle

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