Gnuplot altitude/groundSpeed graph

speed.plt

#!/usr/bin/gnuplot
set terminal png font './LiberationSans-Regular.ttf' 15 size 1600,900
set grid

set xlabel ''
set format x ''

set ylabel 'Altitude, m' tc lt 1
set ytics 1000 nomirror tc lt 1
set y2label 'Ground speed, km/h' tc lt 2
set y2tics 50 nomirror tc lt 2

set datafile separator ','

feet_to_meters(x) = x * 0.3048
knots_to_kmh(x) = x * 1.852

set output 'speed.png'
plot 'data.csv' using (feet_to_meters($1)) with lines lw 2 axes x1y1 notitle, \
     'data.csv' using (knots_to_kmh($2)) with lines lw 2 axes x1y2 notitle