Datasheet

set y2tics border # put tics no right side

set key box top left # legend box

set key box linestyle 0

set xdata time # the x-axis is time

set format x "%H:%M:%S" # display as time

set timefmt "%s" # but read in as 'unix timestamp'

plot "LOGTEST.CSV" using 2:4 with lines title "Light levels"

replot "LOGTEST.CSV" using 2:5 axes x1y2 with lines title "Temperature (F)"

Which makes this: