If we ignore air resistance (which we always do), then the
vertical component has no effect on the horizontal component.
What we have is just a football that's moving horizontally at
13 meters per second, and we don't care what it's doing vertically.
The graph of the football's horizontal position vs time, for as long
as it remains in the air until the ground comes up and grabs it is:
-- a straight line
-- the slope of the line is 13 m/s .
-- If we start the clock at the instant the ball is tossed and
call that time=0, then the line passes through the origin.
(We probably don't care where the ball is before it's tossed,
so we can just as well begin the line at the origin.)
