The mark-mark scatter plot (Ballani et al, 2019) is a
scatterplot of the mark values of all pairs of distinct points in X
which are closer than the distance rmax.
The dots in the scatterplot are coloured according to the
pairwise distance between the two spatial points.
The plot is augmented by three curves explained by
Ballani et al (2019).
If the marks only take a few different values, then
it is usually appropriate to apply random perturbation (jitter)
to the mark values, by setting jit=TRUE.