A spanner which is perfectly horizontal.
When in a Flowable, this spanner's end_y position always maps to the same y-axis position as its starting position in the Flowable's local space.
Otherwise, this spanner's end_y position always maps to the same y-axis position as its starting position in document space.
Like its Spanner superclass, this class is a mixin meant to be combined
with GraphicObjects. Because the horizontal guarantees given by this
spanner depend on the position of the GraphicObject starting position,
it is crucial that, when initializing in implementation __init__ methods,
this be initialized after the GraphicObject exists and has its intended
parent properties set.
This differs from a typical spanner in that neither its end_y or end_pos attributes are settable. Only the end_x or end_parent properties may be modified.
— The y position of the endpoint.
— The parent of the endpoint.
— The x-axis length of the spanner.