From Jon Peltier's site
Add a Horizontal Line to a Column or Line Chart: Error Bar Method.
Double click on the resulting error bar, and on the Patterns tab, select a
color and line style for the horizontal line. Here I've chosen a solid red
line. ...
peltiertech.com/Excel/Charts/AddLineHorzErrBar.html
Add a Horizontal Line to a Column or Line Chart: Series Method.
How do you add a nice horizontal line to a column or line chart, to show a
target value, or the series average? The method involves adding a new
series, ...
peltiertech.com/Excel/Charts/AddLineHorzSeries.html
Add a Horizontal or Vertical Line to a Column or Line Chart.
A variation uses an error bar for the straight line, rather than the
connecting line between two points. Add a Horizontal Line to a Column or
Line Chart: ...
peltiertech.com/Excel/Charts/AddLine.html
Column Chart with Horizontal Line Segments.
The values of interest are shown by columns, while the benchmark or target
level is indicated by a horizontal line above each category. ...
peltiertech.com/Excel/Charts/ColumnHorizLine.html
Combining Different Chart Types into a Single Chart.
The vertical red target line and the horizontal black line separating Model
1 from Model 2 are real-life examples of Adding a Horizontal or Vertical
Line to ...
peltiertech.com/Excel/Charts/ComboCharts.html
Add a Vertical Line to a Column or Line Chart: Series Method.
VertizontalLine.zip is a zipped workbook that shows this technique and
others for reliably placing a vertical or horizontal line on your column or
line ...
peltiertech.com/Excel/Charts/AddLineVertSeries.html
Most examples use XL2003 but the techniques are very similar.
For Excel 2007 eamples, Google with:
excel 2007 chart "horizontal line"
best wsihes
