Part 4 of Charles Petzold’s course on XAML focuses on data, first examining the power of data templates in content controls and items controls, and the art of defining visual trees that give a visual dimension to data. With a little shift in perspective, a data template becomes a ViewModel, and allows XAML to form a crucial part in the popular MVVM (Model-View-ViewModel) application architecture. The video concludes with a tour de force data template that creates a color-coded active-data bar chart, including pictorial tooltips.