Are there any guidelines for labeling axes in plots/graphs?
The recommended way, according to the International System of Units (SI), is to write t/s (SI brochure, §5.3; see also this guide, §7.1). And the SI is the only system of units that should be used to report experimental results.
The rationale behind this notation is the following: a quantity is the product of a numerical value and a unit, so that the ratio of a quantity (e.g. time) and the corresponding unit (e.g second) is a numerical value (a pure number), which can be used to label axes, tables, etc.
In any case, don't use brackets around the unit, like in &quot;t [s]&quot;. The reason is that in quantity calculus brackets represent an operator which means &quot;unit of&quot; (not &quot;dimension of&quot;) and should only be used around quantities, not around units. So, for instance, you can write:
[t] = s
but not [s]. For completeness, I also mention that braces are also used to denote the numerical value of a quantity. So, in L = 5 m, we have [L] = m and {L} = 5.

References
For those interested in learning more about quantities, units, their typographical conventions, and quantity calculus, here is a list of references with a few notes:

J C Maxwell, &quot;A treatise on electricity and magnetism&quot;, vol. I, Oxford: Clarendon Press, 1873. Online. Note: Here Maxwell introduces the concept of quantity and the bracket notation.
International Vocabulary of Metrology - Basic and general concepts and associated terms VIM, 3rd edition, JCGM 200:2008. Online. Note: This is the official dictionary of metrological terms where it is possible to find definition for terms like quantity, system of quantities, system of units, etc. On p. 13 there is note which explains the bracket/braces notation.
I M Mills, &quot;The language of science&quot;, Metrologia 34, pp. 101-109, 1997.
B W Petley, The fundamental physical constants and the frontier of measurement, Adam Hilger, Bristol, 1988.
I M Mills, &quot;Physical quantities and units&quot; in Recent advances in metrology and fundamental constants, Proceedings of the International School of Physics E. Fermi, Varenna, 2000. Online.
J de Boer, &quot;On the history of quantity calculus and the International System&quot;, Metrologia 31, pp. 405-429, 1995.
M J ten Hoor, &quot;Quantity calculus for chemists&quot;, Chemistry in action n. 57, 1999. Online
M L McGlashan, &quot;Physicochemical quantities and units: The grammar and spelling of physical chemistry&quot;, Royal Institute of Chemistry, London, 1971.
E R Cohen et al., &quot;Quantities, Units and Symbols in Physical Chemistry&quot;, IUPAC Green Book, 3rd Edition, 2nd Printing, IUPAC &amp; RSC Publishing, Cambridge, 2008 Online
Nayuki, Handling physical quantities in math formulas. This article is a lively little gem on quantity calculus with lots of practical examples.