The "free" way to do this is to create a formula that returns the value of your target line and then add it to the "Show Data" section in the chart expert so it gets plotted as a series of data. If its a bar or area chart, then the series will initially show up as bar/area, but you can then click on it and do "series options..." and force it to become a line.
The non-free way to do this would be to investigate our CRChart add-in, which has extensive support for programmatically set user-lines. Including options to put labels, values on them, etc...
-Dan
DISCLAIMER: I work for the company, threedgraphics.com, that makes this product. This product costs money.