My biggest issue with this plot is that ""Normal" is defined as the average rainfall between 1931 and 1990." Why are we comparing data from 20 centuries to an average across 59 years? Why not compare each year to an average of the whole data set, or even to an average per century? This graph is conveying that on average, Mexico is dryer than average. That statement is obviously contradictory. What it should be saying, I think, is that in the last century, Mexico has been abnormally wet compared to the average of the last 19 centuries.