Wait just a minute. You said it's a distance/time graph, so there's no line on it
that represents speed. The line represents distance.
If the speed is steadily increasing, then as time goes on, more and more distance
will be covered during each unit of time. The distance line on the graph will
curve upward, more and more steeply as time goes on. If you put a ruler against
the curve, then as time goes on, the slope of the ruler gets bigger and bigger,
because the slope at any point on a distance/time graph is the speed at that time.
