Using panel.densityplot with stripplot

But is there a technical reason for making this distinction? It seemed to
me that most panel functions can deal with x and y parameters and that
panel.densityplot could make use of the same mechanisms as panel.violin to
subdivide based on factor y.

The technical reason is a fundamental part of the Trellis design. The
prepanel function determines a bounding box as part of the initial
calculations. Knowing this in advance allows the panel display and the
axis rendering to be completely separated.

Note that the panel.violin plot cannot easily say anything about the
actual heights of the densities, only their shape. The y-axis there
annotates the sample groups.

-Deepayan