Help System (web edition)

MISSLINE=1 (in scatter plots and time series)
When consecutive points are connected (by LINE specification), the
line is normally broken when missing observations are encountered.
By using MISSLINE=1, this convention is overruled and the line
will be drawn continuously by interpolating the missing points.
However, no markings (due to POINT specification) will appear for
missing points.
  P = more information on plotting