This example demonstrates the following concepts:

• Global variables: The mpgData variable is declared outside of the ui and server function definitions. This makes it available anywhere inside app.R. The code in app.R outside of ui and server function definitions is only run once when the app starts up, so it can't contain user input.
• Reactive expressions: formulaText is a reactive expression. Note how it re-evaluates when the Variable field is changed, but not when the Show Outliers box is unchecked.