arrows or graphics below axes?

Hi, All.  I'm trying to make an arrow pointing to a particular value
on the x axis.

So, this is pretty much what I want:

20     210     220     230     240     250     260 ...
                ^
                |
                |

any ideas how to do this?  I tried a few things (like arrows), but
they didn't want to plot outside of the actual "plot" area...

thanks for any suggestions!

greg
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