To add a horizontal line to your chart in Excel 2007, follow these steps:

1. First, create your line chart by selecting your data and clicking on the "Line Chart" option in the "Charts" section of the "Insert" tab.
2. Once your chart is created, click on the data series to which you want to add the horizontal line. This will select the entire series.
3. Right-click on the selected series and choose "Format Data Series" from the drop-down menu.
4. In the "Format Data Series" dialog box, click on the "Line Color" tab.
5. Under "Line Color," choose the "Solid line" option and select a color for your line.
6. Under "Dash type," choose the "Solid line" option.
7. Under "Width," choose a thickness for your line.
8. Under "Transparency," choose "0%".
9. Under "Line Style," choose the "Horizontal line" option.
10. Under "Value," enter the value at which you want your horizontal line to appear (in your case, 80%).
11. Click "OK" to close the "Format Data Series" dialog box.

Your horizontal line should now appear on your chart at the specified value. You can adjust the line's appearance by repeating these steps and changing the line color, thickness, and other settings as desired.