The horizontal axis represents a calendar year starting on March 21 and ending on the next March 21.  The vertical axis represents the relative length of day.  Notice that the slope is steepest at the equinoxes and shallowest at the solstices.  The change in length of day is rapid at the equinoxes, and slow as molasses at the solstices.
(Accuracy independently verified by John Seward.)
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