Hi Minsu,

Yes, you can definitely select multiple data series in Excel. Here's how:

1. First, select the chart that you want to format. You can do this by clicking anywhere on the chart.
2. Once the chart is selected, you should see a new tab appear in the ribbon called "Chart Tools". Click on this tab.
3. Within the "Chart Tools" tab, you'll see several options, including "Design", "Layout", and "Format". Click on the "Format" tab.
4. Within the "Format" tab, you'll see several options for formatting your chart, including options for changing the line width, color, and style of your data series. To select multiple data series, simply hold down the "Ctrl" key on your keyboard and click on each data series that you want to select.
5. Once you've selected all of the data series that you want to format, you can use the formatting options in the "Format" tab to make changes to all of the selected data series at once.

That's it! By following these steps, you should be able to select multiple data series in Excel and format them all at once. Let me know if you have any other questions!