Real-time plot from serial port

[Larz60+]

without digging into your code, I'd guess that this is some sort of delta issue where two readings in a row are equal
i thought perhaps you could study some of the following code to see how they handle:

package 1:
https://pypi.org/project/oscilloscope/
github: https://github.com/scientifichackers/oscilloscope

package 2:
https://pypi.org/project/pydatascope/
https://sourceforge.net/projects/pydatascope/

more: https://pypi.org/search/?q=oscilloscope