To go straight down, you need to duplicate the x coordinate with the two different y values.
stairs() automatically does the coordinate duplication, but it does it for all the points, which is not what you want, so you will need to duplicate the coordinate yourself.
Or you could add a second x coordinate that is marginally larger than the place you want to plunge; you would only be able to see the difference compared to duplicated coordinates if you zoomed way way in, but there are some cases such as interp1() where duplicated coordinates give you problems.
