The slope of the lines indicates how big an impact different incomes have. Contrast India and Nigeria, for example. A Nigerian needs a lot more to have the same incremental in increase in satisfaction as an Indian. Interestingly, Germany, Russia and India seem to be about parallel, even though the income ranges are different. Is that just coincidence? Or is there an underlying cause? Also, how well does the question itself translate among different languages and societies? Can the same concept be put across effectively?