trendlines

[jowens]

https://altair-viz.github.io/gallery/poly_fit.html

"How about running 1-year-mean with a translucent variance band (like this: https://altair-viz.github.io/gallery/line_with_ci.html)"

[jowens]

https://altair-viz.github.io/user_guide/transform/regression.html#user-guide-regression-transform