year temp label1 year humidity label2
 1990 30   hot    1991 100      wet
 1992 25   warm   1992 83       dry

plot 'weather.dat' using 1:2 with points with labels, '' using 4:5 with points with labels

plot 'weather.data' using 1:2 with points, '' using 1:2:3 with labels

plot 'weather.data' using 1:2 with points, '' using 1:($2+1):3 with labels

... title "some line title"...

... title column(COLUMN_NUMBER) ...

plot 'weather.dat' using 1:2 title column(3) ...

plot 'weather.dat' using 1:2 title column(3) w line lt 1 lc 1 lw 1