Executes one ploticus script line.
Script lines can contain only these directives: #proc, #procdef, #endproc, #clone and #saveas. They cannot contain flow-of-control directives and do not undergo a variable-evaluation pass, so variables cannot be referenced, and select statements (etc.) that reference data field names should use one at sign (@) instead of two (@@).
ploticus variables can be accessed by getvar, described below.
An alternative is to use execscript (described below) to execute an entire script file. execline and execscript cannot be used in the same application.