There is a tail effect from the last presidency over the first 100 days. Better to use the first two years.
I am more interested in the stock market returns over their entire term as President. The first 100 days is a short window and the underlying market directly before they took office all are different (growth period, recession, etc.). I will note that 75.1% bounce under Roosevelt is a very special case. His predecessor (Hoover) had a policy that an economy based on capitalism would self-correct -- claiming the excesses would wash themselves out of the market. Hoover believed that the market situation only impacted profits, not employment and maintained a laissez-faire policy where the government basically did nothing to address the depression. Roosevelt came in promising the government would effectively prop up the markets, provide employment, help the banks, and get government fully involved. Wall Street investors were very receptive to this message and the market recovered greatly in his first 100 days starting on March 4, 1933. The markets trended up under FDR until the 1938 "recession" where the market crashed again by 50% -- but still stayed above the 1932 depression lows.
I think this is the main reason Bernanke didn't raise rates in his tenure. He was an expert in Depression era Fed history and the collapse in 1938 was due in part to the Fed raising rates since they believed the depression was over. Some economic historians refer to it as the ghost of '37.
There is an angle in the original chart also It starts from inauguration day as if the market hadn't known who was the President for 10 weeks already
I think the lesson here is no matter who is elected buy the market on Inauguration Day and sell no later than 100 days later.