Figure 1. Pourbaix diagram of iron (Source)
The line between $\ce{Fe}$ and $\ce{Fe^2+}$ is there because the potential $\ce{Fe/Fe^2+}$ is $\pu{-0.44 V}$. The same applies for $\ce{Fe^2+/Fe^3+}$ with $\pu{0.77 V}$. But $\ce{Fe/Fe^3+}$ is $\pu{-0.03 V}$, but it's not on the graph as a line.
How do I know which species have a line, and which don't? And if I'm drawing a graph, how do I know that I also shouldn't include this line?
Pourbaix diagrams are very useful to identify the nature chemical species in different pH environment.
The figure contains additional information on the stability of water. The lines at 1.23 V and 0.00 V represent the redox chemistry of water. At 1.23 V water is being oxidised to oxygen. At 0.00 V the water is reduced to hydrogen. As labelled in Fig.1, there are 3 regions where behaviour of water is represented. The middle region is where the water in "stable state".
Note: These voltages are obtained at 298 K. In order to derive these lines, you can use Nernst Equation (where gradient is: -RT/nF, as the both reactions involve 2 electrons the gradient is the same in both cases.)
Figure 1: Stability of water.
Now moving on to iron. Overlapping water behaviour and iron as in Fig.2. (The dashed lines - redox chemistry of water). In the case of water, knowing the pH given you can predict the behaviour of iron. For the sake of understanding how to utilise the Pourbaix diagrams.
Assuming you are analysing alkaline lake (high pH: 10-12) and assume there is not much oxygen content (to utilise also the y-axis, low oxygen level - E decreases). In Fig.2, the region of your interest is pH 10-12 and since low oxygen, and region around 0.00 V. The Pourbaix indidicates that at these conditions you can have some amounts of Fe(OH)2, Fe(OH)3.
Figure 2: Analysis of Iron species in 1 M of water (298 K).
To see different behaviour of other species: http://www.crct.polymtl.ca/ephweb.php
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