After ploting that chart a small bae was shown. (I used CartesianChart)
I do not know what it is. Consequently I did not find any way to remove it!
What is its name and how can I remove it?
I though it is DataTooltip! But, that was something else. I googled my issue, but I didn't find anything.
It's only visible in debugging. This answer shows how to remove it for debugging. The actual application won't have it.
To disable it, uncheck the Enable XAML Hot Reload under Tools -> Options.