[ferret_users] how to draw a straight line on a plot?

[Zibiao Zhang <zzhang@xxxxxxx>]

Hi All,
How can you draw vertical or horizontal straight lines on a plot? for
example, the I range is 1:500, how can I draw a vertical line at I=100?

This may be a super easy stupid question, but really bothered me for a
while. Please help!

Zibiao