d3/d3-voronoi

D3-voronoi is a JavaScript library that takes a set of points on a flat surface and divides that surface into regions, so that every spot in a given region is closest to the point it belongs to. This division is called a Voronoi diagram. If you have ever seen a map that looks like it is cracked into irregular shapes, where each shape centers on a city or data point, that is what this tool creates. Behind the scenes, the library uses a classic algorithm by Steven J. Fortune to draw the boundaries between these regions. You give it a list of coordinates, and it hands back the polygon shapes, the lines between them, or a related structure called a Delaunay triangulation, which is a mesh of triangles connecting nearby points. You can also ask it to find the nearest point to any spot you click. People building interactive charts and maps use this to make small data points easier to hover over or click. For example, The New York Times used it in a baseball visualization so that even tiny dots on a scatterplot had a large, easy-to-hit surrounding area. It is also handy for positioning labels so they do not overlap, or for figuring out which points are neighbors of each other. One notable thing about this project is that it is deprecated. The authors recommend a newer, faster, and more robust replacement called d3-delaunay. That said, this older library still works and offers a straightforward set of tools for anyone who needs to reason about the space between points.