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.