The phrase "second diagram" refers to a visual representation or drawing that comes after or is the second one in a series or set. It is used to specify a particular diagram when there are multiple ones and helps avoid confusion. Full definition
second diagram represents a circuit in which Bailey has the possibility to light the bulb. (religion-online.org)
By contrast the second diagram does not place digital books in opposition to, or at a distance from, print books. (theliteraryplatform.com)
This second diagram takes into account the ways in which different types of digital book build on, or clearly draw inspiration from, different types of print book and their conventions [i]. (theliteraryplatform.com)