The speed that the line moves is determined by the speed that you draw it. Try drawing very slowly and watching for problems in difficult levels. To make your line move straight, make sure its ends are even. A diagonal angle will cause the line to move off horizontal. This is critical if you are creating an arch. If one end of the arch higher than the other, the line will be unbalanced. The ends of an arch must be the same length in order for it to move straight across the screen. Make sure the end of your line does not contain extra movements, which may cause it to move off course.