An acceleration-time graph plots signed acceleration on the vertical axis against time on the horizontal axis.
The ordinate gives instantaneous acceleration. A horizontal line represents constant acceleration, including a line on the time axis for zero acceleration. A changing ordinate represents non-uniform acceleration.
The sign of acceleration describes the direction in which velocity is changing. It does not by itself state whether the object is speeding up: speed increases when velocity and acceleration have the same sign and decreases when their signs are opposite.
The change in velocity over an interval is the signed area between the graph and the time axis,
An initial velocity is required to reconstruct the velocity-time graph. Area under an acceleration-time graph does not directly give displacement.
For calculus-based courses, the same signed-area relationship may be written
This integral notation is an extension of the graphical meaning. IB Physics and AP Physics 1 questions can use geometric signed areas without requiring calculus.