Creating a 3rd axis (or coordinate) in Excel chart
 

I'm using Excel 2003, and am trying to create a simple line chart with a
"Time" x-axis with hours 06:00 through 06:00 (24 hours), and a "Contaminant
Level" y-axis. That's the easy part.

The product that the chart is based on has a "Lot Number" and Time of
Production. Each lot takes approximately 1.8 hours, and often has gaps in
the production time. I am (unsuccessfully so far) trying to create a second
"Time" x-axis at the top of the chart (not a second chart line), based on
the Lot Number Production Time, relative to the "Time" on the bottom of the
chart. Is this even possible in Excel?