Why N$_2$ is a non-absorbing species in the spectrum of the Earth? The transmission spectrum of the Earth atmosphere is like that (Kaltenegger &amp; Traub 2009):

As you can see, you can find many absorption lines related to some components of Earth's atmosphere: H$_2$O, O$_2$, CO$_2$, O$_3$, CH$_4$... But the nitrogen, N$_2$, is not detected anywhere in the spectrum. Why N$_2$ is not detected, but O$_2$, for example, is?
As your question is based on the plot you posted, I suggest you to look for a lower wavelength range of the atmospheric electromagnetic absorption. A quick search in google gave me this paper, which says:

The importance of molecular nitrogen as the most abundant species in the Earth's atmosphere is evident. The strong absorption bands in the range 80–100 nm shield the Earth's surface from theextreme ultraviolet (XUV) part of the solar radiation.

As you can see, the absorption bands of diatomic nitrogen resides in the &lt;1000 nm, which is in the extreme left of your plot, shadowed by its scale (1 micrometer = 1000 nanometers).