Linewidth and Line Style

The linewidth and linestyle properties let you control the thickness and visual pattern of a line in a Matplotlib chart. These are simple but powerful customisations that make your plots clearer and more professional.

Linewidth is measured in pixels. The default linewidth is 1 pixel, which produces a thin line. If you set linewidth to a number greater than 1, the line becomes thicker — the larger the number, the thicker the line. For example, linewidth=2 gives a line twice as thick as the default.

Linestyle controls the pattern of the line. You can set it using a string value. The accepted options are:

• "solid" — a continuous line (this is the default)
• "dotted" — a line made of dots
• "dashed" — a line made of dashes
• "dashdot" — a line that alternates dashes and dots

These two properties are used together inside the plot() function. For instance, plt.plot(x, y, linestyle="dashed", linewidth=2) would draw a dashed line that is 2 pixels thick.

The textbook illustrates this with a complete program (Program 4-3). It uses two lists — height and weight — for persons aged 8 to 16. The data is first stored in a Pandas DataFrame with columns named "height" and "weight". Then the plot is created with the following specifications:

• x-axis: weight (labelled "Weight in kg")
• y-axis: height (labelled "Height in cm")
• line colour: green
• marker: * (asterisk)
• marker size: 10
• title: "Average weight with respect to average height"
• line style: dashed
• linewidth: 2 …