Explanation:
The graph shows the public debt in %GDP on the left axis with the so-called "real interest rate (r) - economic growth (g)" or "r-g" on the lower axis. Positive values on the bottom axis mean, that the growth of interest rates is higher than the economic growth. Or with other words, the countries are in danger to go bankrupt, they are excessively borrowing money at an unsustainable pace.
Negative values on the other hand mean, that the economic growth is larger than the rise of the interest rates due to borrowing money. Djibouti, Somalia and Eritrea by the way are missing from the graph.