I think they mean the variable width of the graph's columns. If you watch it as the graph moves, there are gaps at every 2 columns.

I don't understand though the thing about font priorities.
And also, would that just change all fonts? Unless you mod the font to only have the braille characters..

No, you've got it set up right. Many people will have graphs where each character rectangle has open circles for the unused braile dots in the character block.

Here's an example.