In mathematics a type of graphing involves finding slope. The formula for slope can be said to be slope equals the second y coordinate minus the first y-coordinate over the second x-coordinate minus the first x-coordinate.
With this in mind, the slope of an object accelerating at a constant rate would look like a straight and horizontal line on an acceleration vs time graph. Because the acceleration is at a constant rate there would be no up and down sloping to record. Instead it would go straight along in straight line that does not show any deviation at all.