Here is a way to do it with some functions to help adjust the positions of the margins in the plot:
#!/usr/bin/env gnuplot
### n: change this parameter to equal the number of data sets to be plotted
n = 3
# t: top margin in pixels
t = 75.0
# b: key height in pixels (bottom margin)
b = 300.0
# h: height of output in pixels
h = 150.0*n + t + b
### define functions to help set top/bottom margins
top(i,n,h,t,b) = 1.0 - (t+(h-t-b)*(i-1)/n)/h
bot(i,n,h,t,b) = 1.0 - (t+(h-t-b)*i/n)/h
### first set up some basic plot parameters
set term pngcairo enhanced size 800,h font 'FreeMono-Bold,14'
set output 'bigkey.png'
set title 'Big Key Plot'
set ylabel 'Y Axis'
set multiplot layout (n+1),1
### First plot
# change only plot command here
currentplot = 1
set tmargin at screen top(currentplot,n,h,t,b)
set bmargin at screen bot(currentplot,n,h,t,b)
unset key
unset xtics
plot sin(1*x) title 'Line 1', \
sin(2*x) title 'Line 2', \
sin(3*x) title 'Line 3', \
sin(4*x) title 'Line 4', \
sin(5*x) title 'Line 5', \
sin(6*x) title 'Line 6', \
sin(7*x) title 'Line 7'
### Middle plot
# copy and paste this code to make more middle plots
currentplot = currentplot + 1
set tmargin at screen top(currentplot,n,h,t,b)
set bmargin at screen bot(currentplot,n,h,t,b)
unset title
replot
### Last plot
# change only plot command here
currentplot = currentplot + 1
set tmargin at screen top(currentplot,n,h,t,b)
set bmargin at screen bot(currentplot,n,h,t,b)
set xlabel 'X Axis'
set xtics
replot
### Last (key) plot
set tmargin at screen bot(n,n,h,t,b)
set bmargin at screen 0
set key center center
set border 0
unset tics
unset xlabel
unset ylabel
set yrange [0:1]
plot 2 t 'Line 1', \
2 t 'Line 2', \
2 t 'Line 3', \
2 t 'Line 4', \
2 t 'Line 5', \
2 t 'Line 6', \
2 t 'Line 7'
unset multiplot
Here is the result:
The things you have to adjust manually are the number of plots and the size of the bottom margin, which is where the key gets drawn. The comments generally say where you have to change things.
I put the key in a plot by itself so that it would be easy to keep the sizes of the other plots the exact same. Plotting the number 2 when the y range is 0 to 1 keeps the lines from showing in the key plot.
This is mostly based on my answer here, which is why I have the plots all touching. You can adjust the top() and bot() functions with an additional margin parameter if you want a gap between the plots.
Best Answer
To adjust the length of the samples:
(default is 4)
To adjust the vertical spacing of the samples:
(default is 1.25)
and (for completeness), to adjust the fontsize:
(default depends on the terminal)
And of course, all these can be combined into one line:
Note that you can also change the position of the key using
set key at <position>
or any one of the pre-defined positions (which I'll just defer tohelp key
at this point)