To shade in part of a graph in Microsoft Excel, you can use the "area chart" feature. Here are the steps to follow:

1. Enter your data into Excel and create a line graph as you normally would.
2. Right-click on the line in the graph that represents the area you want to shade in.
3. Select "Format Data Series" from the drop-down menu.
4. In the "Format Data Series" window, select the "Fill" option.
5. Choose the color you want to use for the shading.
6. Under "Fill", select "Solid fill".
7. Under "Transparency", adjust the slider to the desired level of transparency. This will allow the line to still be visible underneath the shading.
8. Click "Close" to apply the changes.

Your graph should now have the desired shading above the line. If you need to adjust the shading or the line, simply repeat these steps until you achieve the desired effect.