Thanks to Catherine Smith for this link. The American Bible Society have produced a simple synopsis of the Passion Narratives in the Gospels in English (CEV) and Spanish (VP):
The Passion and Resurrection Narratives from Matthew, Mark, Luke and John
It is aligned at the verse-by-verse level rather than the line-by-line level so its use is primarily to get a general impression rather than to do more detailed work. And only major parallels are shown. One helpful feature is that you can press a button on each page to generate a PDF of the Synopsis in question.