Click the plot to select it.

From the Plot menu, select Line. Alternatively, use the context-sensitive options in the Context Panel.

Select Line Width. The Set Line Width dialog opens.

Increase or decrease the line width using the arrow buttons, or enter a line width greater than 0.

Click OK.

Click the plot to select it.

From the Plot menu, select Line. Alternatively, use the context-sensitive options in the Context Panel.

Select Line Width. The Line Width dialog opens.

Enter a line width between 1 (narrow) and 50 (wide), or change the line width value using the Slider.

Click OK.

When using the plot and plot3d commands, line width can be set using the thickness=n option. The value of n can be any positive integer. For details, see plot[options] and plot3d[option].