I've been encouraged to go ahead and post anyway: an interesting challenge.
So, here's an artist's impression of what it might have looked like (names have been changed to protect the idiotic):
And here's the wall as it looks now (like the map of the ocean in The Hunting of the Snark, it may be a bit short on detail):
My gardener, who is full of facts, some of which might even be true, informs me that these scrawls are boundary demarcation lines agreed between teenage street gangs.
I'm going away now to imagine the inter-gang Summit conference, and to ponder on the global geopolitical parallels, if any. Also to get my supper.