plot lastx=0.0,lasty=0.0,integ=0.0,integ_init=0.0, “sample.txt” u 1:2 w l , “” u 1:(dx=$1-lastx,integ = ($0==0 ? integ_init : integ+dx*($2+lasty)*0.5), lastx=$1, lasty=$2, integ) w l