in a graph, why might the data be represented in horizontal line segments?
Lots of reasons. It depends upon what is plotted in the graph. You can get horizontal line segments in cases of
(1) distance vs. time during a peiod when a vehicle is not moving
(2) velocity vs. time during a period when a vehicle is not accelerating
(3) money in an account vs. time during a period of no deposits or withdrwals
(4) when you only have data on the amount of the dependent variable that corresponds to specific ranges of the independent variable. Such graphs are called histograms. For example: average income vs. age for age intervals of 20-30, 30-40, 50-60 etc.
so that it is clear too read and understand