It's difficult for us to automatically graph asymptotes for a variety of reasons. However, we hope to have this feature in the future! In the meantime, it's possible to create an asymptote manually.
Start by graphing the equation of the asymptote on a separate expression line. To change its styling to a dotted line, click and long hold the icon next to the expression. The style menu will appear.
Pro-tip: For repeating asymptotes, try using lists to save time.