While working on the 51 Line Charts for my Opioid Crisis data visualization, I ran into an issue with generating all 51 charts at the same time: The data was either in 51 rows stacked and I couldn’t access the dates effectively, or 306 rows unstacked and I had 6x as many objects as I needed when I did a join.
The solution was to use D3.nest. Nesting allows elements in an array to be grouped into a hierarchical tree structure; think of it like the GROUP BY operator in SQL, except you can have multiple levels of grouping, and the resulting output is a tree rather than a flat table. The levels in the tree are specified by key functions.
When I used the state as a key, it grouped each state’s data together in a tree.
A few useful tools and resources for understanding d3.nest: