The distance vs. time graph of a car moving at constant speed should be a straight line. Why do the data points in the graph plotted from observing the motion not fall right on the line? A graph with 7 points and a line segment representing a straight line. Horizontal axis is labeled Time, in seconds, and extends from 0 to 7. Vertical axis is labeled Distance, in meters, and extends from 0 to 50. Approximate coordinates of plotted points are begin ordered pair 1 comma 8 end ordered pair comma begin ordered pair 2 comma 13 end ordered pair comma begin ordered pair 3 comma 21 end ordered pair comma begin ordered pair 4 comma 28 end ordered pair comma begin ordered pair 5 comma 35 end ordered pair comma begin ordered pair 6 comma 39 end ordered pair comma begin ordered pair 7 comma 49 end ordered pair. Line segment has positive slope, and it begins at the point begin ordered pair 0 comma 0 end ordered pair, and it ends at the point begin ordered pair 7 comma 49 end ordered pair. The measurements are inexact. Data points can never look like they fall right on a straight line. The distance is not exactly proportional to time even if the speed is exactly constant. The distance and time are proportional only if the correct units of meters and seconds are used.