A connector is made perfectly horizontal by dragging a shape upward; and then the connector is shortened by dragging the shape inward.
When you want to change the length or angle of a connector, do so by repositioning the shape it's attached to. Click the Play button to see some animated examples.
Make a connector perfectly vertical or horizontal
It can be frustrating sometimes to get a connector to be perfectly horizontal or vertical between shapes. Key point: Since the connectors attach at hot points, they sit at the angle the hot points dictate. Sometimes the hot points are straight across from each other and there's no problem. But if they aren't, and the connector is slightly angled, you have to adjust the position of the shapes.
Change the connector's length
To shorten or lengthen a connector, move the shape it's attached to. Moving the connector by itself pulls it away from the shape (a green dot shows an unattached end).