Ian Johnson is the author of Tributary - an open source live-coding environment designed for prototyping data visualizations with D3. He also co-organizes the SF Bay Area d3 Meetup and makes educational D3 videos.
Data Visualization with D3
Categorical scales adjust the positioning of items. They are useful when a certain number of items must spaced evenly in a region. Ian create a bar chart to demonstrate using these categorical scales.