Looks like a regression line between April 1 and May 1. The line moves around between the 11th and the 16th from the year ~850 until ~1950, but never crosses i

Looks like a regression line between April 1 and May 1. The line moves around between the 11th and the 16th from the year ~850 until ~1950, but never crosses it until ~1950. The current average is about 6 days earlier (April 5th).