To undelete legend items in Microsoft Excel, you can follow these steps:

1. Right-click on the chart and select "Select Data" from the drop-down menu.
2. In the "Select Data Source" dialog box, click on the "Hidden and Empty Cells" button at the bottom left corner.
3. In the "Hidden and Empty Cell Settings" dialog box, select "Show data in hidden rows and columns" and click OK.
4. Now, you should be able to see the deleted legend items in the chart. Simply click on the checkbox next to the legend item to bring it back.

As for an alternative to deleting legend entries, you can try renaming them to something more descriptive. For example, instead of and , you can rename them to "Original Data" and "Trendline". This way, the legend will still be informative without cluttering the chart. To rename a legend entry, simply double-click on the legend item and type in the new name.