If it was the graphics card that was the problem, then when you're mirroring to other displays it would show up there as well. However overall, the problem sounds suspiciously like the LCD is damaged. If you have Apple Care, Apple may cover it. Otherwise you're best bet, price wise (Apple 'aint cheap to fix stuff), is to go to a computer repair shop or consider a new computer. Because usually these problems get worse over time.
I had a similar problem with my Macbook Air (same model) but my case was far more pronounced and the screen changed colour '''severely''' with the computer needing a new screen. Again, Apple will charge a lot. But Apple Care will take care of the problem if you have it.