These visualizations very simply compares the average annual increase in GDP growth between the two parties, starting from four different points in time, measured in real dollars on a per capita basis. Clearly, in the larger metrics (After WW1 forward and GD & Forward) Democrat presidents hold a substantial advantage in GDP growth results. However, this difference shrinks quickly after WW2, and reverses on the Reagan & Forward measurment.
The reason for this is the enormous distortions that occurred during the 1929-1945 period; Hoover (R) took huge negatives, while Roosevelt (D) got both the reversion to the mean and the heightened WW2 production. Rightly viewed, these are both outlier events that should be discounted due to their nonrepeatability. (Hoover took negatives of 10%, 7%, and 13.5% for 1930-1932; for comparison, the worst three year period during the recent Great Recession was a mere +1.2%, -1.2%, and -4.55% for 2007 through 2009). Although it was a deep recession, there are four three-year periods in the US history OUTSIDE the Great Depression that were worse (1913-1915, 1907-1909, 1892-1894, and 1865-1867).
Simply put, over the last 32 years, there has been no serious differential between the ability of the two parties to deliver GDP growth.