Add option for horizontal line at 0

[stefan-mueller]

Hi @AakaashRao, thank you very much for the development of this package. It's very useful for our work. I have one improvement suggestion: would it be possible to add an option for a dashed line that crosses the y-axis at 0 to see more easily and clearly which model specifications cross/do not cross 0 and are thus statistically significant?

I guess this could be done fairly easily by adding geom_hline(yintercept = 0) and by allowing users to specify the linetype, colour, and size. What do you think?