The metal abundance increases to the center of a galaxy, because more stars are formed, as in the peripheral regions (spiral arms). This diagram illustrates this is not due to lack of very distant stars are included in the database.
The Milky Way center is about 37,000 ly from the Sun, where the graph theoretical significant metallicity increase would be watching. On the other hand, the stars move in the course of their lives away from the center (small cool stars with a higher life expectancy), so that, for example, the sun (with a high metallicity) in the peripheral regions (spiral arms) is.