key

The layout of certain plot styles (column-stacked histograms, spider plots) is such that it would make no sense to generate plot titles from a data column header. Also it would make no sense to generate axis tic labels from the content of a data column (e.g. using 2:3:xticlabels(1)). These plots styles instead use the form using 2:3:key(1) to generate plot titles for the key from the text content of a data column, usually a first column of row headers. See the example given for spiderplot.