A very large library of premade charts for web. Probably the largest I've seen. Less customizable than chart.js, and D3 is more of a rendering library than a charting library.
I'm surprised by your claim of it being less customizable than chart.js, it has been very flexible from my experience.
It could rival d3 with a lot of customization and a worse DX from what I've seen: It's essentially a good amount of defaults, but you can override and replace essentially anything.
