Streamline the UI by hiding elements

This is a simple function that you are certainly going to need if you build even a moderately complex application. Those of you who have been doing extra credit exercises and/or experimenting with your own applications will probably have already wished for this or, indeed, have already found it.

conditionalPanel() allows you to show or hide UI elements based on other selections within the UI. The function takes a condition (in JavaScript, but the form and syntax will be familiar from many languages) and a UI element, and displays the UI only when the condition is true. This is actually used a couple of times in the advanced GA application, and indeed in all the applications, I've ever written of even moderate complexity.

We're going to show the option to smooth the trend graph only when the trend graph tab is displayed, and we're going to show the controls for the animated graph only when the animated graph tab is displayed.