In comments to my previous post, Robert requested the unsmoothed data from Graph 3. Here it is. GPDI is plotted against the Capital Gains Tax Rate.
Since the Capital Gains Tax Rate (X-axis) is quantized, the result is columns of data. Compared to the smoothed version, there is little change in either the slope or intercept of the best fit straight line. R^2 is, of course, much lower.