Another graph that comes up frequently in GMAT graphics interpretation questions is the line graph. Line graphs display information that occurs over time or across graduated measurements and are particularly effective in highlighting trends, peaks, or lows. Typically (but not always), the x-axis displays units of time or measurement (the independent variable), and the y-axis presents the data that’s being measured (the dependent variable).
1. Basic Line Graphs
The line graph in the figure shows the garbage production for three cities for each of the four quarters of 2011. You can tell from the graph that Plainfield produced more garbage in every quarter than the other two cities did, and it is evident that all three cities produced less garbage in Quarter 3 than they did in the other quarters.
2. Scatter Plots
Line graphs are extensions of scatter graphs, or scatter plots, which display the relationship between two numerical variables. These graphs display a bunch of points that show the relationship between two variables, one represented on the x-axis and the other on the y-axis.
For example, the scatter plot in the figure plots each city’s population on the x-axis and its garbage production on the y-axis. Scatter plots show you trends and patterns. You can figure out that, generally, a direct or positive relationship exists between a city’s population and the amount of garbage it produces. The graph indicates that this is the case
because the data points tend to be higher on the y-axis as they move to the right (or increase) on the x-axis.
You can also surmise that of the 20 cities listed, more have fewer than 200,000 people than have greater than 200,000 people. That’s because the graph shows a greater number of points that fall to the left of the 200,000 population line than to the right.
Scatter plots also convey trend and pattern deviations. The GMAT may provide a scatter plot with or without a trend line. The trend line shows the overall pattern of the data plots and reveals deviations.