Increasing frequency of wars

The graph is from the paper titled "The Frequency of Civil Wars" by Harrison and Wolf (The Economic History Review, August 2012). An draft (2008) is here. From the conclusion: 

The evidence suggests that, normalized by the number of countries in the world, the risk of war is lower today than at the end of the nineteenth century. Normalized by the number of planets we have to share, however, it is of the same frequency (if not intensity) as during World War I. There has been a steady upward trend in the number of bilateral conflicts over 130 years.