The dotted line in the diagram is a spectrum, produced by a Fourier analysis. It shows how the O16/O18 isotope ratio had cyclic variation, as we go deeper down a core drilled from the ocean floor. The X axis labels give frequencies, and the right hand Y axis gives how much the isotope ratio departed from normal.
The solid line in the diagram shows how much insolation is predicted from Milankovitch theory. The left hand Y axis gives how much the insolation should vary from normal.
This diagram is from
Eccentricity forcing of Pliocene-Early Pleistocene climate revealed in a marine oxygen-isotope record, S. Clemens and R. Tiedemann, Nature vol. 385, 27 February 1997, pp. 801-804The article discusses many fine points of this and other graphs, such as the 202,000 year peak, which is thought to be a second harmonic of the 404,000 year peak. It mentions things which a spectrum cannot show, such as the way the 23,000 year isotope cycle and the 23,000 year insolation cycle are synchronized (phase locked).
For our purposes, the important point is that the dates for the Milankovitch cycles are computed by astronomers. The dates for the dotted (isotope) line are measurements by geologists. The fact that the 41,000 year peaks line up so beautifully is evidence that the geological dates are accurate. The article (published in one of the most prestigious scientific journals in the world) didn't even mention the accuracy of the geological dating. Contrary to the claims of some Creationists, most geological dates are accepted without question.