$g.line(60,20,60,100,$black) draws a line from
(60,20) to (60,100) using the color $black.
We can control the thickness of the line and the end-cap.
The cap determines what happens at the end of a line.
We draw three thick black lines. Each has thickness 15.
It seems that gd does not support end caps.
Please inform ajcumming@gmail.com if you know different.