Feeling "like you can hit" was "l'esprit du jour" in ballparks that Saturday. Six other MLB players also belted grand slams. Zunino's was the seventh, a new single-day record. "It's officially the grandest day in Major League Baseball history," wrote MLB.com.

That record-setting Saturday was part of a record-setting month in a record-setting year for Major League Baseball, mirroring the Dow Jones Industrial Average's record-setting streak to numerous all-time highs. In June, players hit more home runs than in any previous month in history. And on September 19, hitters broke the record for home runs hit in a single season.

The previous single-day grand slam and single-month home run records were set in May 2000, the year of the previous record high for single-season home runs — and a year in which the stock market also set an all-time high.

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Home runs weren't the only records set in baseball this year. Pitchers also threw more "immaculate innings" (striking out a side on nine consecutive pitches) than ever before. The New Yorker wrote, "it's true that the increased tendency to swing for home runs comes with an additional likelihood that one will miss: Strikeouts have also spiked to record rates."

Our chart plots 147 years of social mood as reflected by a PPI-adjusted index of U.S. stocks versus two indicators of baseball performance. The bottom line plots average home runs per MLB game. Homers have trended roughly parallel to the Dow/PPI, but the relationship is not perfect. Notable divergences surround some of the major peaks and troughs.

The middle line in the chart plots average home runs plus strikeouts per game, a new baseball metric which I dubbed the "Swing for the Fences" indicator, or SWAT — not an exact acronym, but it's catchy and close enough. This performance index has had an even tighter relationship with the Dow/PPI, especially since the 1940s. Today, stocks, SWAT and average home runs per game are at new all-time highs.