Phenograms (also known as 'traitgrams') are visual methods for "mapping the estimated state of a continous character on the tree", and often assume a Brownian motion model of evolution.
Adding a kernel density next to a phenogram can be useful specially when we are interested in one ore more clade whithin a larger phylogeny.
For example, see this figure from Muñoz et al., 2025 created using similar code as the one below: