Last year, some groups of students in their community (社区) made a survey about "How do people spend their free time?" The following diagrams show some results of it:
Diagram (1): Hours for people of different ages who surf the Internet in a week.

Diagram (2): Different days people go to the library. (library attendance)

Diagram (3): Different programs people like.

根据上面三个图表内容，选择正确答案。

已知y+a与x+b（a、b为常数）成正比例．

（1）y是x的一次函数吗？请说明理由；

（2）在什么条件下y是x的正比例函数．