Vega-Lite is a grammar for interactive graphics primarily used for analysis. Altair is a visualization library in Python that is based on this grammar.
With Altair, you can spend more time understanding your data and its meaning. Altair’s API is simple, friendly and consistent and built on top of the powerful Vega-Lite visualization grammar. This elegant simplicity produces beautiful and effective visualizations with a minimal amount of code.
Jim Vallandingham just put up a useful introduction to the library if you’re looking to get your feet wet.
I do very little visualization-wise with Python since my current toolset typically covers my bases, but this has me curious.