Two keywords control whether or not every point in the plot is marked with a symbol, pointinterval (short form pi) and pointnumber (short form pn).
pi N or pi -N tells gnuplot to only place a symbol on every Nth point. A negative value for N will erase the portion of line segment that passes underneath the symbol. The size of the erased portion is controlled by set pointintervalbox.
pn N or pn -N tells gnuplot to label only N of the data points, evenly spaced over the data set. As with pi, a negative value for N will erase the portion of line segment that passes underneath the symbol.
Copyright 1986 - 1993, 1998, 2004 Thomas Williams, Colin Kelley
Distributed under the gnuplot license (rights to distribute modified versions are withheld).