Without your data and your chart actually running, we can't really tell, but at a guess it's the X axis that is giving the problem - I suspect it isn't moving to reflect the data change.
To be honest, the MS chart isn't really meant for real-time display of rolling data, you would be better off using something that is. Have a look at this:
Live Charts[
^] - it's free, and the source is available on GitHub if you need to customise it.