I have nine x-axis labels whose names are: 4, 5, ..., and 12, as seen in the picture below:

However, the graph only shows eight labels (contains a half 4). How can I display all x-axis labels on the graph on my screen?
My thought is reduce the distance between each label. But from the wiki, I did not find a related method.
EDIT
When I run the app on an iPhone with a smaller screen, there are fewer labels displayed.