Adding quantile regression lines

There is another very useful kind of regression, quantiles. Drawing them under the ggplot2 package it's not challenging; it has a whole quantile dedicated function, geom_quantile(). Drawing them using ggvis and plotly is also possible, but demands way more code.

This recipe draws 20 percent, 40 percent, 60 percent, and 80 percent quantile regression lines in a diamonds' carat versus price scatterplot. Drawing are amde using ggplot2 and plotly respectively.