Conversion between gnuplot's coordinates systems

Question

TL;DR

Is there a way to align corners of different plots in a multiplot setup?

Is there a way to convert axis coordinates to screen c

Answer 1

In order to achieve an exact overlap, I think that an option would be to adjust the margin manually:

set terminal pngcairo
set output 'fig.png'

eps_h = 0.1
eps_w = 0.1

set multiplot
unset key

set lmargin at screen 2*eps_w
set tmargin at screen 1 - eps_h
set rmargin at screen 1 - eps_w
set bmargin at screen eps_h

plot cos(x) w lp lc rgb 'dark-red'

set border ls 4;
unset xlabel
unset ylabel
unset tics

plot 2*cos(x+pi) w lp lc rgb 'royalblue'

which gives:

Answer 2

You can fix the margins to keep the automatic size after the first plot as follows (see also https://stackoverflow.com/a/19132068/2604213):

fix_margins = 'set margins '.\
    'screen GPVAL_TERM_SCALE * GPVAL_TERM_XMIN/(1.0*GPVAL_TERM_XSIZE), '.\
    'screen GPVAL_TERM_SCALE * GPVAL_TERM_XMAX/(1.0*GPVAL_TERM_XSIZE), '.\
    'screen GPVAL_TERM_SCALE * GPVAL_TERM_YMIN/(1.0*GPVAL_TERM_YSIZE), '.\
    'screen GPVAL_TERM_SCALE * GPVAL_TERM_YMAX/(1.0*GPVAL_TERM_YSIZE)'

set samples 20
set xrange [-pi:pi]; set yrange [-2:1]
set xlabel "x"; set ylabel "y"
unset key

set multiplot
plot cos(x) w lp
eval(fix_margins)
unset tics; unset xlabel; unset ylabel
plot 2*cos(x+pi) w lp
unset multiplot