I'd say 'across the year' hints strongly at sporadic events etc, whereas 'throughout the year' speaks more of consistent results. The comparison with the second year and the last year supports plural rather than singular. But 'the year' means a particular/specified year or the one which is.
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When historical reporting is included, the column for this year's survey will be thinner, and the column for last year will appear behind it in grey. Unless you're talking about chinese (or persian) new year, the name of the celebration isn't new year but new year's eve, and it happens. 'a year' can be any year without any specification.
In that full version, each occurrence of the word year.
