on from the entire data at once, rather than
from each individual group of data. This choice makes ridgeline plots
look more uniform, but the density estimates can in some cases look
quite different from what you would get from <code>geom_density</code>
or <code>stat_density</code>. This problem can be remedied by using
<code>stat_density</code> with <code>geom_density_ridges</code>. This
works just fine, we just need to make sure that we map the calculated
density onto the <code>height</code> aesthetic.</p