Function Breakpoints Limited to One Axis Adjustment

I'd like to be able to limit the start and end breakpoints in my Function object to only be adjustable in the y axis.

My current solution is to use a slider in front of the start and end points in presentation, and use the slider value to set the start / end point's y value. Aside from being inelegant, the only problem I've found with this approach is that it's possible to move a midpoint "behind" the [invisible] slider and not be able to click on it again.

Is there a better way to do this?