This question was asked a few years ago by someone else:

http://groups.google.co.uk/group/microsoft.public.excel.charting/msg/f335a4916f3440fc

Basically, I am using a line chart for a 'wrong' purpose. X axis values
are actually categories and data points are not joined by lines. I am
doing this to produce a chart such as this:
http://conceptwheel.servehttp.com/chart.gif .

The answer given in the previous thread suggested faking numbers on a
scattergraph to create the effect. But that was in 2004 - I assume
there must be, at least by now, a better way to do this! This is
actually quite a common graph in scientific literature so someone must
have made a plugin or script, surely!

Looking forward to hearing from someone,

Paul