[FR] Areaplot should have a step form - OR - create separate line segment property

[BioTurboNick]

I'd like to generate an areaplot that can have a step-function form for the line.

More generally, it might be useful to have a separate property to determine how adjacent values are joined? Instead of having a separate series type.

linear (default) one or more smoothed options: cubic spline? steppost steppre stepmid