4.2 matplotlib

Big ideas

• import matplotlib.pyplot as plt. The usual recipe: prepare x and y data, call a plot function, add labels, then plt.show() (or savefig).
• Plot types: plt.plot(x, y) lines, plt.scatter(x, y) points, plt.bar(labels, heights), plt.hist(values, bins=10), plt.pie(sizes, labels=...).
• Dress it up: plt.title, plt.xlabel, plt.ylabel, plt.legend() (needs label= on each series), plt.grid(True), colors and markers ("ro--").
• Several series: call plot twice before show. Several panels: fig, axes = plt.subplots(1, 2) then axes[0].plot(...).
• A good chart answers one question: pick the type that shows it (trend over time = line, category comparison = bar, distribution = histogram, relationship = scatter).

Code you should know

import numpy as np
import matplotlib.pyplot as plt

x = np.linspace(0, 10, 50)
plt.plot(x, np.sin(x), label="sin")
plt.plot(x, np.cos(x), "r--", label="cos")
plt.scatter([2, 5, 8], [0.9, -0.9, 0.99], color="black", label="samples")
plt.title("Waves")
plt.xlabel("x")
plt.ylabel("value")
plt.legend()
plt.grid(True)
plt.show()

A chart appears in the picture pane.

import numpy as np
import matplotlib.pyplot as plt

fig, axes = plt.subplots(1, 2, figsize=(9, 3.5))
axes[0].bar(["Mon", "Tue", "Wed", "Thu"], [3, 7, 4, 6], color="seagreen")
axes[0].set_title("Study hours")
rolls = np.random.randint(1, 7, size=300)
axes[1].hist(rolls, bins=6, edgecolor="white")
axes[1].set_title("300 dice rolls")
plt.tight_layout()
plt.show()

Two panels.

Worked example

Common mistakes

• Calling plt.legend() without giving each series a label.
• Forgetting plt.show() in a script, so nothing appears.
• Plotting x and y of different lengths (ValueError).