How do I depict an sp-hybridised carbon with two double bonds?
$$\ce{R=C=R}$$
If I just leave it out, like I would for a normal $\mathrm{sp^3}$ or $\mathrm{sp}$ carbon, it's difficult to see that it's there.
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In chemical structure diagrams, carbon atoms that are bonded to at least two other atoms are usually left unlabeled. The carbon atom is indicated by the angle in the bonds.
However, according to the Graphical Representation Standards for Chemical Structure Diagrams (IUPAC Recommendations 2008), a carbon atom with two double bonds should be explicitly labeled:
(…), any carbon atom with two identical collinear bonds should always be explicitly labeled, to remove the possibility of the two bonds being misinterpreted as one long bond.
The use of a dot in place of an explicit carbon atom label is acceptable in allenes and related molecules with three consecutive carbon atoms. It is not acceptable to use a dot to represent a carbon atom when either of its adjacent atoms is other than a carbon atom.
When a dot is used to represent a carbon atom, it must be depicted clearly and unambiguously. The dot should be large enough to be clearly seen by the reader, and must be visibly separated from the adjacent bonds.
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$\begingroup$ And then there’s also the variant of one continuous line and two non-connecting double-bond lines which is probably badly inacceptable by IUPAC ;) $\endgroup$– JanOct 2, 2016 at 22:26