horizontal grid line in a particular latitude

[Satyaban BishoyiRatna]

Dear Grads-user
    I have ploted a time-latitude cross-section of vorticity.Here x-axis with time and y-axis with latitude.The latitude value varies from 15 N to 20 N. But I would like to draw a horizontal line in a desired latitude(say at 17.5).Please give me sugession to do this job.
    Thanking you.

with best regards
Satyaban
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