Band Charts fill a polygon between two high and low lines. The colour of the polygon changes depending on which line Y1 or Y2 is higher.
This variation on Band charts uses Spline interpolation (smoothing). This JS chart type can be used to draw thresholds, a fill between two lines or areas of interest on a chart.
If you have data where Y1 is greater than Y2 always, you’ll get an envelope effect. Great for rendering confidence intervals, error margins or Bollinger Bands!