Does it really matter? Many schools do it in different order anyway.
The reason Graphs is the first chapter in most textbooks (not just Cambridge) is because it's the first topic according to the syllabus. The order of topics in the syllabus (not necessarily the order schools follow) is:
1) Graphs
2) Complex Numbers
3) Conics
4) Integration
5) Volumes
6) Mechanics
7) Polynomials
8) Harder Extension 1 topics
My guess is that graphs works on mostly previously developed skill and knowledge. Teachers are more pointing out things you might have never thought of before. Complex numbers, well you're teaching new stuff. So I'm thinking, best start of with what students know most about and then get to teaching them new stuff.