@Preben Alsholm Your second command is exactly what I search :) Thank you very much.
But, I have another problem now.I have two plots including same labels. I combined them by command "plots:-display([plot1,plot2]);" Label "Time t" appears, but the label including subscript don't.
It is bizarre that it works in the above example, but it doesn' t work in some examples. (I can' t share all of the code since the code will be published in the journal paper) But in shortly, labels:
( where q:=2, labels=["Time t",typeset('w'(q,t))] )
I noticed that it is originated from "w" . when I changed it with "m" or "a", It worked. Is there a bug?
Do you have any ideas about it?