The following code should do the trick, it will work by using worksheet column designation, so if there is no X, then it will plot with implicit X.
string wkbname$="Book1";
range wks1=[wkbname$]1!; // 1st sheet
for (int ii=3; ii<=wks1.ncols; ii++)
{
range y1 = [wkbname$]1!wcol(ii);//take LN from 1st sheet
string strColLN = y1[L]$;
plotxy [wkbname$](1:end)!(?, $(ii)) plot:=200 ogl:=[<NEW>]!;
//result plot is the active window
page.LongName$=strColLN$;
}
See this link for the XYRange notation of (?,col) for the implicit designations.
http://wiki.originlab.com/~originla/wiki/index.php?title=LabTalk:Range_Notation#Special_Notations_with_XYRange_using_.23_and_.3F_for_X