Hi esong_98,
Thank you for your post in our forum. And we will try our best to help you if we can do something.
Per your description, we test on our end, and it works well. So, we would like to suggest you can double click the Horizontal Axis to open the Format Axis window > click Axis options to see the Bounds, like the below screenshot.
However, if you can’t change the line chart, we would suggest you can create a new workbook to see whether the problem persists, this can help us eliminate the problem of the Excel client app.
If the newly created workbook works well without any problem, we think the problem may be related to the specific workbook. And then you can sign in to Office online in InPrivate or Incognito browser and click Excel to open the file to insert a line chart, and then you can check whether the problem disappears.
Finally, may I know your Excel version?
Best regards,
Cindy