This function allows its user to define events with mouse clicks on
  the current plot via the locator function.  The
  arguments n and type are the usual parameters of the
  locator function.  If EID is not missing, then
  n = length(EID).
On exit from locator, suppose the user defined m
  events.  If EID was missing, then the output data frame will
  contain m events. However, if EID exists, then the
  output data frame will contain length(EID) events, and both
  X and Y will be NA for events
  EID[(m+1):n].  The na.omit function
  can remove rows with NAs.