To quote explainxkcd.com:
Each light in this Christmas scene is represented by its electromagnetic spectrum, which shows in a graphical form how much energy is radiated by each wavelength of light.
These graphs plot the intensity of all visible radiation. Infrared and ultraviolet are partially plotted also, represented by black. It starts with longer wavelengths on the left (infrared), continues with visible light in the middle from red to blue, and ends with ultraviolet at the right.
(The image posted with this comment is what we'd be looking at, realistically.)
