Before we get into understanding the relationship and graph between speed and time, its better to have an idea of these two concepts individually.
In a speed-time graph, speed is always shown on the Y-axis or the vertical axis and time are shown on the horizontal or X-axis.
What is the acceleration? The rate at which speed changes is defined as acceleration. In day to day parlance, acceleration refers only to increase in speed. When an ad for a vehicle says 0 to 100 in 5 seconds, this commonly refers to what is referred to as acceleration.