Hello Boris Kodrič, Thank you for bringing this to our attention. We appreciate that you've taken the time to post your concern here in Microsoft Community Forum.We understand that you’re having trouble editing data in Excel. Let’s check on this together and find ways to address this matter.
I have replicated the issue and it would be best if you select the " edit data in Excel " instead of edit data so that your excel sheet would open on a bigger size and you don't need to resize it every time you edit the graph and it would still save all changes on the file.
We look forward to your response on the results. If you have other clarifications about this matter, you can get back to us by replying to this post.
Sincerely, Juhn Jac,Consumer M365 Forum Moderator