What causes vertical (blinking) lines on screen: GPU or display issue?

Hi,

My MacBook Pro all of a sudden started to have one vertical line (blue). Over time (weeks) more lines appeared, some static, some blinking. Now I am wondering: what is the cause of this? Is it a GPU issue, or is something broken in the display? If it is a GPU issue, I can replace it (at my work we have a soldering station to do these kind of things), if it is a display issue, I might get a replacement here from iFixit.

What can I do to determine what the problem is?

Thanks in advance.

Kind regards,

Sjoerd