In the Year of the Boarand Jackie Robinson refers to the transformational events that take place for Shirley over the course of the twelve months of her life that the book covers. It represents the two sides of Shirley's journey—her beginnings in China, and her adaptation to American culture.
The year in which the story occurs—1947—is the Year of the Boar in Chinese culture, and its presence in the title is a shout-out to the parts of Shirley and her experiences in this book that are Chinese or influenced by her Chinese identity.
On the flip side, Jackie Robinson represents the American components of Shirley's story in this book. He becomes Shirley's hero, and insofar as he represents the possibility she has in America (more on that over in the "Symbols" section), his presence in the title clues us into the fact that American culture is a pretty major player in this book as well.
Jackie Robinson's appearance at the end of the book and in the novel's title shows how, for Shirley, 1947 goes from being just the Year of Boar (representative of her home in China) to being both the Year of the Boar (she retains her Chinese roots) and the year of Jackie Robinson as she becomes Chinese-American.