Final answer:
The type of data that must be plotted on a graph for the slope of the line to represent density is altitude on the horizontal axis and air density on the vertical axis. The slope of the line between two points on the graph gives information about the change in air density per unit change in altitude.
Step-by-step explanation:
The type of data that must be plotted on a graph for the slope of the line to represent density is the altitude on the horizontal axis and the air density on the vertical axis. In the given example, the line graph represents how thin the air becomes as you climb a mountain, with altitude measured in meters above sea level and air density measured by the weight of the air in a cubic meter of space. The slope of the line between two points on the graph gives information about the change in air density per unit change in altitude, allowing us to determine the density gradient or how quickly the density of air changes with altitude.