set <axis>label '<text>'
The setting xlabel sets the label which should be written along the x-axis. For example,
set xlabel '$x$'
sets the label on the x-axis to read ‘’. Labels can be placed on higher numbered axes by inserting their number after the ‘x’; for example,
set x10label 'foo'
would label the tenth horizontal axis. Similarly, labels can be placed on vertical axes as follows:
set ylabel '$y$' set y2label 'foo'