Create data:
d = data.frame(a=rnorm(10),b=rnorm(10))
Plot boxplots of a defined width:
my_width = 0.5
boxplot(d, boxwex=my_width)
Add lines for global transmissivity using another boxplot (green)
global_tr = 0.3
extra_d = data.frame(a=global_tr,b=global_tr)
boxplot(x = extra_d, at = c(1,2), add=T,col=3, boxwex=my_width, border=3)
... or using segments (red)
x = 1:2
segments(x0=x-0.5*my_width, x1=x+0.5*my_width,y0=0.3, y1=0.3, col=2)