Given the emission spectrum of a "white" light source, its CIE General Colour Rendering Index Ra (see Wikipedia for a descriptive explanation) is a single number with a maximum value of 100. A diagram for multiple emission spectra would be their Ra values plotted on a line. (It is possible to have negative values of Ra, although the lowest I have ever seen is -17.)

You can alternatively draw uv plots, if you have access to raw data. Similar plots are commonly used in more updated colour rendering metrics, such as CQS, TM-30-15 and Rf. They are visual aids to show colour shifts under test light sources.