It's showing a live plot. Data is received and plotted live. It is also saved and exported when asked as a PDF. I suppose you update the log file then redraw the file on screen.
One use case I have is my solar controller (Victron) that spews out serial data every second and I can, if I want, prod the plotter to update every 20s. The image viewer picks up that the file has changed and displays the latest version.
Absolutely horrible solution on the solid state media, I had intended on coming back to it and inserting a memory buffer but here were are 3 years later.
And it's not a Pi, it's a cheap NUC. But misery loves company!