set grid y
set yrange [30:70]

set label sprintf("Channel %g",channel+1) at graph 0.95, 0.1 right

f(x)=a*x+b
fit f(x) datafile index channel u 2:1 via a,b

set xrange [a*30+b:a*70+b]

#fit doesn't converge the other way round
a=1/a
b=-b*a

plot datafile index channel using 1:2 title "bla", f(x)

pause mouse

#weiter bei linker
if(MOUSE_KEY==1) channel=channel+1
#zurueck bei rechter
if(MOUSE_KEY==3) channel=channel-1
#Ende bei mittlerer
if(MOUSE_KEY==2) channel=channel+99

if(channel<21) reread