I think I'd heard of that term (synesthesia) before but I didn't realize it applied to me. And there's a name for the kind of 3-d number line I always picture too: it's called a number-form. I've asked a variety of people if they picture sequences like I do, and have found very few who do. And I think there's only been one person (another entp) who had a number line as interesting as mine.
Do you have any kind of synesthesia (see: http://en.wikipedia.org/wiki/Syne