To turn off a data series on a chart in Excel, follow these steps:

1. Click on the chart to select it.
2. Click on the data series you want to turn off. This will select the entire series.
3. Right-click on the selected series and choose "Format Data Series" from the context menu.
4. In the Format Data Series pane that appears on the right side of the screen, find the "Series Options" section.
5. In the "Series Options" section, you'll see a checkbox labeled "Plot Series On." Uncheck this box to turn off the data series.
6. Click anywhere outside of the Format Data Series pane to close it.

The data series you turned off should now be hidden from the chart. To turn it back on, simply follow the same steps and check the "Plot Series On" box.

Alternatively, you can also use the Legend feature in Excel to turn off a data series. To do this, simply click on the legend entry for the series you want to turn off. This will select the entire series, and you can then press the "Delete" key on your keyboard to remove it from the chart. To turn the series back on, you can either re-add it to the chart or use the "Undo" feature in Excel.