A recap of some key crossfilter tasks: creating dimensions, grouping by dimension, and group reduce; see lines 21-40 for assignment prompts

Go to line 92. This block of code demonstrates a basic implementation of d3.brush. A brush is a UI element that allows the user to select and highlight a region, giving the user the ability to create a filtering range.

You are not expected to learn to master brushes yet. Take a look at the example and try to understand its basic implementation. Between lines 110 and 113, write a block of code to update the other charts, based on the results of the filtering operation.

The function drawUserType should draw (and update/redraw) a pie chart showing the breakdown of registered vs. casual users. However, as it's currently implemented, it doesn't support updates. This is because the "update/exit" part of the "enter/update/exit" pattern is not implemented. Refractor the code between lines 146-165 to account for this.

For extra practice, complete the drawUserGender function.