D3

D3 gives you complete control over custom data visualizations, building charts and interactive graphics from scratch that no charting library offers out of the box. It's not a charting library. It's a visualization toolkit that binds data to DOM elements and lets you manipulate them directly, the most-starred data visualization library on GitHub. ISC license (permissive). Created by Mike Bostock, now maintained by Observable. D3 powers visualizations at the New York Times, NASA, Uber, and thousands of other organizations.

The power is in the primitives: scales that map data to pixels, axes, shape generators, geographic projections, force simulations, hierarchy layouts. You compose these to build exactly what you need. The Observable notebook ecosystem has thousands of examples you can fork.

Fully free. No paid tier for D3 itself. Observable (the company) offers a paid collaboration platform, but D3 the library is and always will be free. Solo developers: free, but expect a steep learning curve. D3 is not drag-and-drop. Small to large teams: if you have a developer who knows D3, the results are unmatched. If nobody on the team wants to learn it, use Chart.js or ECharts.

The catch: D3 is powerful precisely because it's low-level, and that's also why it's hard. Making a simple bar chart takes 50 lines of code instead of a config object. The learning curve is real, weeks, not hours. For standard charts, use a higher-level library. Reach for D3 when nothing else can do what you need.