Ability to add multiple horizontal and vertical lines to `scatterPlot`

[j-andrews7]

E.g. take a vector of numerics as input. Being able to add vertical lines as well would be nice. Could do it with a x.add.lines and y.add.lines parameters along with matching linetype and color for each axis.

Horizontal lines are already available in yPlot, and vertical lines make less sense there and would be tougher to implement.