Or, an even more direct way to measure the rate of change.
Taking a starting value of 387 for the beginning of 2009 (a favorable for your view's interpretation of the data in the graph), and an ending value of 395 for the end of 2012, shows a gain of 8 ppm for that 4 years. I make that an average gain of 2 PPM per year, compared to your immutable law of 1.5. Do your numbers dispute that somehow?
-edit- I added an apostrophe to denote possessivity (is that a word?).