Vega-Lite. A “high-level grammar of interactive graphics”. Part of the Vega project, which provides a mechanism for creating declarative visualizations by defining them using JSON. Vega-Lite is particularly interesting to me because it makes extremely tasteful decisions about how data should be visualized—give it some records, tell it which properties to plot on an axis and it will default to a display that makes sense for that data. The more I play with this the more impressed I am at the quality of its default settings.