The Line Cap (Butt, Rounded, or Square) of a figure can be altered by clicking on the Stroke Tab in the Sidebar, as shown below.
To alter the Line Cap Type, click on one of the choices (Butt, Rounded, or Square) under the line cap heading. The selection will appear in blue as a visual indicator of the currently active line cap type.
For example, the image below shows what a Line would look like with a Butt, Rounded, and Square Line Cap.
Note: A line cap change will be seen instantaneously when a stroke figure is selected. If no figures have been selected, the line cap change will apply to all subsequently created stroke figures.