Q:

Draw front arrow over image

I have the following image using the code

set terminal png
set output 'plot.png'
set xlabel "GC (%)"
set ylabel "Proportion of genome"
set sample 1000
set xrange[0:100]
set yrange[0:]
set boxwidth 1
set style fill solid
set key off
set style line 1 lt 1 lc rgb "#0000FF" lw 3
set style line 2 lt 2 lc rgb "#32CD32" lw 3
Cauchy(x,xo,wi) = (1./pi) * wi / ((x - xo)**2 + wi**2)
plot 'compositionGC.txt' w boxes, 0.368187*Cauchy(x, 41.4226,1.72758) +    0.631813*Cauchy(x, 51.8272, 0.464711) ls 2
set yrange[0:GPVAL_Y_MAX]
set arrow from 47.2,0 to 47.2,GPVAL_Y_MAX front ls 1
replot

I have everything I need except that I want a vertical blue arrow separating the two peaks at 47.2 in the x-axis in the front. I cannot get it to work.

A:

Do not change the yrange and replot.
Use this arrow command before the initial plot:
set arrow 1 from 47.2, graph 0 to 47.2, graph 1 front lc "blue"

