Multiple Axes
Multiple X- and Y-axes are often used when one needs to: 1) compare data point values in different units, for example, Celsius against Fahrenheit degrees, or inches of mercury (mmHg or inHg); 2) show data from the different ranges on the same plot, for example, absolute stock price changes and sales volume (price will be in dollars and volume in millions of dollars); 3) show data measured in different units on the same plot, for example, gross domestic product (GDP) volume and GDP growth rate (GDP will be in billions and rate in percent).
