So, I found a really cool minimalist Back to the Future pop art poster by an artist called Jamie Bolton. Here it is:

The idea is that it uses dots and lines to depict the order of Marty & Doc's adventures through time throughout the three brilliant movies. The poster is presumably inspired by the simple diagram Doc Brown draws up in BTTF2 to explain the concept of an altered timeline to a confused Marty.

So, if you look back up at the pop art poster, I'll talk you through it. The first dot on the left represents 1885, which isn't travelled to until BTTF Part 3. The second dot represents 1955. The third dot represents Marty's "present", 1985 (we'll get to the one underneath it in a sec), and the fourth represents 2015. Now, the dot underneath 1985 represents the "altered" 1985, which was made different by the DeLorian's intervention in 1955 (in this "skewed" 1985, Biff Tannen, the series' main protagonist, is a multi-millionaire and the town of Hill Valley has declined into a criminal hotbed where entropy and anarchy reign supreme). Once you understand this, you can start to see how the curved lines represent the DeLorian hopping forward and backward through time. Pretty clever, eh? Except (and this is where I get nerdy and pedantic) it's wrong.

In Back to the Future Part 1, Marty goes back in time to 1955, meddles around with stuff, and then goes back home to 1985- only it's not the same 1985 that he left. Indeed, the whole point of the movie was that he had tweaked a crucial turning point in his parents' lives, significantly changing them for the better. In this "improved" 1985, Marty's family is happy and successful, and Biff who in the original 1985 was George's overbearing, asshole supervisor is now the family bitch.

So really, this: 

Should look like this:

In my "fixed" version, the top right dot represents the original 1985, which Marty leaves to travel back to 1955 (the dot to the top left), and after permanently altering it, he then travels forward to the 1985 in the "new" timeline. Which means that the rest of the poster should look like this: