Double column plot in Gnuplot multiplot

you should add a plot command before the last two functions and maybe a unset multiplot at the end. This should work. Or do you want the single plot and the multiplot both visible?

plot sin(x) title "this should be a single plot"

set multiplot layout 2,2 title "Those are the multiplots"

set title "A!"
plot sin(x)

set title "B!"
plot cos(x) not

set title "C!"
plot tan(x) title "tan"

set title "D"
plot tan(0.5*x) not

unset multiplot

Edit: (manual setting of sizes, origins and margins)

### Multiplot layout
reset session
set multiplot title "These are five plots"
set ytics 0.5

set margins 5,5,2,8 # l,r,b,t

set size 1,0.5
set origin 0,0.6
set title "top plot"
plot sin(x) title "this should be a single plot"

set size 0.5,0.5

set origin 0,0.3
set title "A!"
plot sin(x)

set origin 0.5,0.3
set title "B!"
plot cos(x)

set origin 0,0
set title "C!"
plot sin(2*x)

set origin 0.5,0
set title "D"
plot cos(2*x)

unset multiplot
### end of code

Result:

Addition:

Just for fun, maybe it is useful to you or somebody else. With a few lines you'll have an easy way to set your layout within a matrix with a few numbers stored in $Layout. I hope it is self-explaining.

Code: (edit: simplified)

### easy configurable multiplot layout
reset session

# row column height width
$Layout <<EOD
1 1 1 1
1 2 2 2
1 4 2 1
2 1 1 1
3 1 1 4
EOD

stats $Layout u ($1+$3):($2+$4) nooutput  # get max number of rows and columns
MPRows       = STATS_max_x - 1
MPCols       = STATS_max_y - 1
r(i)         = word($Layout[i],1)
c(i)         = word($Layout[i],2)
h(i)         = word($Layout[i],3)
w(i)         = word($Layout[i],4)
MPGridX      = 1.0/MPCols
MPGridY      = 1.0/MPRows
MPSizeX(i)   = MPGridX*w(i)
MPSizeY(i)   = MPGridY*h(i)
MPOriginX(i) = MPGridX*(c(i)-1)
MPOriginY(i) = 1-MPGridY*(r(i)+h(i)-1)

SetLayout = 'i=i+1; \
             set origin MPOriginX(i), MPOriginY(i); \
             set size   MPSizeX(i)  , MPSizeY(i)'
 
set multiplot
    set linetype 1 lc rgb "red"
    i=0
    @SetLayout
    plot sin(x)
 
    @SetLayout
    plot cos(x)
 
    @SetLayout
    plot x**3
   
    @SetLayout
    plot x**2
   
    @SetLayout
    plot sin(x)/x
unset multiplot
### end of code

Result: