Figure A1.2

(b) shows the relationship during the 1990s between the percentage of Americans who own a cell phone and the average monthly cell-phone bill. This scatter diagram reveals that as the cost of using a cell phone falls, the number of cell-phone subscribers increases.

A time-series graph measures time (for example, months or years) on the x-axis and the variable or variables in which we are interested on the y-axis. Figure A1.2

(c) shows an example. In this graph, time (on the x-

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axis) is measured in years, which run from 1980 to 2016. The variable that