Here's how you can add a standard deviation to your trendline in Excel:

1. Select your chart by clicking on it.
2. Click on the "Chart Elements" button (the plus sign icon) that appears next to your chart.
3. From the drop-down menu, select "Trendline" and then "More Options."
4. In the "Format Trendline" pane that appears on the right-hand side of your screen, select the "Options" tab.
5. Under "Forecast," you'll see an option for "Confidence Interval." Select the drop-down menu next to it and choose "1 Standard Deviation."
6. You should now see two additional lines on your chart, one above and one below your trendline, representing one standard deviation from the trendline.

If you want to customize the appearance of these lines, you can do so by selecting them and using the formatting options in the "Format Data Series" pane that appears on the right-hand side of your screen.