Syntax
plot FOO using x:y:z:color smooth zsort with points lc palette
The intended use is to filter presentation of 2D scatter plots with a
huge number of points so that the distribution of high-scoring points
remains evident. Sorting the points on z guarantees that points with
a high z-value will not be obscured by points with lower z-values.
Limited to plot styles "with points" and "with linespoints".