What is this graffiti on the walls in Portal 1?

I just started playing Portal, and found these scrawls on the wall (somewhat hidden). The first two images were in the first level with turrets, and the second was a couple levels after.

It appears represent the companion cube as a friend (symbolized by hearts around the pictures), and the cake.

What does this graffiti mean (if anything), and who wrote it?

The graffiti was written by:

Doug Rattmann, the only known surviving employee at the Aperture Science Enrichment Center. He was suspicious of GLaDOS and was prepared when GLaDOS decided to attack the facility. He survived the attack but was trapped inside the facility.

Meaning of the graffiti:

Rattmann escaped a neurotoxin attack caused by GLaDOS, but by going into hiding got cut off from his supply of schizophrenia medication. This made him become delusional and unstable. The graffiti, simply put, is the expression of a madman. He has an obsession with the Weighted Companion Cube, which he thinks can talk. The graffiti also foreshadows that GLaDOS is not telling the truth about the tests.