Ubuntu regular is a variable width font, different characters have different widths. Therefore its not possible to draw a continuous straight vertical line through a specific column since the position of that column depends on which characters exist in the lines.

The vertical line is drawn at the position as if all the lines contained all spaces.

When you use a "mono" font is its monospaced, ie all characters have the same width, so column 72 of a line of spaces is the same as a line of `a`s or `z`s or anything and it works.

For those looking for a specific font, `DejaVu Sans Mono Book` is a good choice.