shiny/inst/examples/04_mpg

This example demonstrates the following concepts:

• Global variables: The mpgData variable is declared outside the shinyServer function. This makes it available anywhere inside shinyServer. The code in server.R outside shinyServer 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 ticked.