st: how to graph vertical bars on scatter plot

[mganz <mganz@hsph.harvard.edu>]

Can anyone point me in the right direction:
I have a data on a probability mass function that I can graph as scatterplot. 
Suppose I want to draw 2 vertical bars (representing some confidence interval) 
such that the bars reach from the x axis to the curve.  Any suggestions?

Thanks,
Michael

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