The pointinterval and pointnumber properties of a line type are used only
in plot style linespoints. A negative value of pointinterval or pointnumber,
e.g. -N, means that before the selected set of point symbols are drawn a box
(actually circle) behind each point symbol is blanked out by filling with the
background color. The command set pointintervalbox controls the radius of
this blanked-out region. It is a multiplier for the default radius, which is
equal to the point size.
