Why can you hide horizontal but not vertical grid lines?

You can hide horizontal grid lines, by spacing them 0.0 pixels apart which gives you a grill of vertical lines rather than a grid. However, you cannot hide the vertical grid lines using the same technique. Is this a bug or a feature?

It is an inconsistent behavior. I don’t know if it is a bug (show the grid) or a feature.
By definition, a grid of size 0.0 means that every point belongs to the grid so any point would snap to it. So I think that don’t display the grid on the axis where it is zero is the normal behavior.
I have to look why when the x size is zero the grid disappears. I suspect that the grid is drawn using a double loop and when the exterior loop doesn’t have any increment (zero length) the loop doesn’t exists (no grid is drawn). On contrary if the inner loop increment is zero the outer loop can draw the grid.

Thanks - I’ve added this one to the bug tracker.