Mays, Aaron: lgBA difference per season.

Willie Mays and Hank Aaron have very different lgBA in baseball-reference.com under Advanced Batting.  Presumably this is caused by they're having different home parks.  Mays had the advantage every season 1954 through 1967 except 1963, which was about even.  Aaron's Braves moved from Milwaukee to Atlanta in 1966.  Aaron had the advantage every season 1969 through 1973 except 1968 was about even.

The biggest difference for either player was for Aaron 1972 and 1973 when Mays played mostly for the Mets whose home park was Shea Stadium: 13 and 12 points.  Mays topped out with a nine point edge in 1957, his Giants last season in New York where they played at the Polo Grounds.

This only matters for something like the simple thing I was trying to do: figure the percent above the league average, which is becoming an increasingly elusive concept.

Year  Aaron-Mays
1954 -0.008
1955 -0.006
1956 -0.006
1957 -0.009
1958 -0.007 Mays Seal Stadium San Francisco
1959 -0.008 Mays Seal Stadium San Francisco
1960 -0.002 Mays Candlestick Park
1961 -0.005
1962 -0.003
1963 0
1964 -0.002
1965 -0.004
1966 -0.001 Aaron Atlanta
1967 -0.001
1968 0
1969 0.003
1970 0.007
1971 0.008
1972 0.013 Mays Shea Stadium New York and Candlestick Park
1973 0.012 Mays Shea Stadium New York