[CAUTION: I am not a teacher:take the advice and or corrections offered in this post at your own risk.
If you doubt the information, please get a qualified opinion from one of the teachers on these forums.]
Your question is a little unclear to me, are you referring to a mathematical figure or a diagram in a textbook of some kind?
If that is the case I believe you should say--
"I see a line in figure nine."
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