Why can't I color tick marks by private or public?

The little Warning to check the console scares me that appeared when I was in a infinite loop. What does it mean?

Your web browser has what's called a "developer console." It's a place where Observable (or any other website) can print information. It's usually used by the developer when developing a website to help with debugging their code. On Chrome on a Mac, you can open it with option-command-I. It sounds like Observable was trying to tell you something.

Can you move cells to different places in the notebook? For example could I move my filtered data above my tick chart?