Actually that is the entire timeline. 🙂 Maybe you were looking for the dopesheet view? The timeline view can be useful if you have the dopesheet on the right or left of the viewport and you'd like a second timeline below your skeleton, to avoid moving your mouse very far just to scrub the timeline.
Spine uses a pane system for laying out views, so we can't prevent you from making the timeline viewer taller than is really useful. I suppose it may be less confusing to center it vertically rather than have that dark space.
