How can I represent a car that is accelerating in speed-time graph?
By giving the graph a positive gradient.
In a speed-time graph, the slope of the graph represents the acceleration of the car.
Mathematically one can say that the slope of a distance-time graph gives the speed/velocity of the object.
While in a speed-time graph the slope gives the acceleration of the object.
Giving the graph a steep, positive gradient implies that it has a rapid, positive, acceleration. Conversely, giving the graph a negative gradient shows a negative acceleration- the car is braking!