I am facing a problem of using ymin and ymax functions.

It always returns me a error like, *Error* eval: unbound variable - ymin.

My script is in the following.  Could anyone shine some light on it?  Thanks a lot.

analysis('tran ?stop "0.4m"  )
desVar(        "sen" 900m      )
desVar(        "parameter_ini" 200n      )
desVar(        "parameter_dc2" 210n      )
desVar(        "parameter_dc1" 230n      )
paramAnalysis( "parameter_dc2" ?start 200n ?stop 400n ?step 100n
   paramAnalysis( "parameter_dc1" ?start 200n ?stop 400n ?step 100n)
)
temp( 27 )
paramRun()
selectResult( 'tran )
plot(getData("/Out2") getData("/Out1") getData("/Out1_no_fb") getData("/Out2_no_fb") )
min_value1 = ymin (VT("/Out1") )