There was a syntax error, xcols was specified instead of xcon.
Adding in the Column() function and Eval() function for the xcol list fixed the JSL
Try this:
dt = Open( "$SAMPLE_DATA/Semiconductor Capability.jmp" );
yCol = {"NPN1", "NPN2"};
xCol = {"wafer", "lot_id"};
// I want to Plot for all x columns
obj = dt << Variability Chart( Y( column(yCol[1]) ), Model( "Main Effect" ), X( eval(xcol) ) );
Also, please use the JSL icon for entering any JSL that is specified in your question and/or response.
Jim