Ellingham Diagram for carbon Consider the following reactions:
$$\begin{align}\ce{C + O2&amp;-&gt;CO2}\tag{1}\\[1em]
\ce{2C + O2&amp;-&gt;2CO}\tag{2}\\[1em]
\ce{2CO + O2&amp;-&gt;2CO2}\tag{3}\\[1.5em]
[(2)+(3)]/2 &amp;= (1)\end{align}$$

Since $G$ is a state function, why aren't the straight lines corresponding to these reactions in the Ellingham diagram concurrent?
I have checked the graph from other sites but found none to be concurrent.
Is there some kind of error in data or extrapolation of the graphs or any other specific reason for this?
You are indeed right, I confirmed that it is the fault in the diagram itself. See the diagram by wikipedia: