Plot the vertices of a texture classification system, on top
of an already drawn texture triangle plot. Also plot the
vertices numbers. See TT.vertices.tbl() and TT.classes.tbl()
for a non graphic, tabular equivalent of the plot.
TT.vertices.plot(geo, class.sys = "HYPRES.TT", fg = NULL,
col = NULL, cex = NULL, font = NULL, family.op = NULL,
adj = NULL, pos = NULL, offset = NULL, blr.tx = NULL,
text.sum = NULL, blr.clock = NULL)