Adding an arrow below the x axis in R plots

Question

I am trying to add arrows marking specific x coordinates below the x axis in an R plot. My x axis is at y=0 and when I try to use negative y-coordinates in arrows, so the arrows will be perpendicular to x axis, I get only the very edges of the arrow plotted (although is some space, e,g where the x-axis label and tickmarks are plotted).

Answer 1

The xpd option can be used in arrows so you can just set your coordinates to be outside your plot region and set xpd to TRUE. For example, assuming xlim = c(0,10) and ylim = (0,10), and you set the x-axis to 0 then

arrows(1.4, -1, 1.4, 0, xpd = TRUE)

draws a vertical arrow pointing up at the x-axis at position 1.4 on that axis.