Why use absorption spectra not emission to identify gas?

It may seem remarkable that we can learn about the composition of distant stars by studying the light they emit. In fact, we can learn a great deal, not only about the chemical elements present, but also about physical conditions. The key is to spread the light out by color, producing a spectrum like the one shown in Fig. 1. This lab explores some of the basic ideas used to analyze spectra.

the answer is:  

line graphs show patterns in data with variables that change over time; whereas, circle graphs show data as parts out of a whole (such as out of 100%).