How to decide which state is on and which is off in a checkbox

There's a program that draws various types of horizontal lines that anchor to other horizontal objects that are usually a little bit tilted.

Some of the lines, by default, are completely horizontal, meaning they have no slope, even though the objects they are anchored to do. Some other lines, by default, get the same slope of the objects they are anchored to.

Although these are the defaults, it's useful to override them in both cases. What would be better, to have a checkbox in the line properties saying "Allow diagonal", or to have a checkbox saying "Force horizontal"? Why?