Example 1:
plot 'function_data1.dat' using 1:3 with points pointtype 5 pointsize 2
replot 'function_data1.dat' using 1:7 with points pointtype 5 pointsize 2
set logscale y
plot 'function_data1.dat' using 1:3 with points pointtype 5 pointsize 2
replot 'function_data1.dat' using 1:7 with points pointtype 5 pointsize 2


Example 2:
plot 'function_data1.dat' using 1:6
replot x

# A few try.
replot 30*x
replot 25*x
replot x**2
#replot (1/2)*x**2
replot 0.5*x**2

# fit example
b=0.0
m=0.0
f(x)=m*x+b
fit f(x) 'function_data1.dat' using 1:6 via m,b
replot 26.5*x-82.7