add.ribbon: Plot time series ribbon for posterior draws
In colemonnahan/sraplus:

Plot time series ribbon for posterior draws

1	add.ribbon(x, z, col, alpha.level, alpha.min = 0, alpha.max = 1)

`x`	Vector of years
`z`	Matrix of values (years by draws)
`col`	The base color without transparency.
`alpha.level`	This is the credible region, e.g. 0.5 will contain 50 shading effect. The transparaceny is proportional to the credible region (wider regions are lighter).
`alpha.min`	Minimum level of shading, for fine control of coloring. Defaults to 0.
`alpha.max`	See alpha.min. Defaults to 1.