Tell Octave to hold the current data on the plot when executing
 subsequent plotting commands.  This allows you to execute a series
 of plot commands and have all the lines end up on the same figure.
 The default is for each new plot command to clear the plot device
 first.  For example, the command

  grhold on

 turns the hold state on.  An argument of off' turns the hold state
 off, and hold' with no arguments toggles the current hold state.