how to add a transparent rectangle to a R boxplot (plot)?

Question

I have a simple boxplot for my data using R ..

boxplot (Error~Code, DataFrame1, xlim = c(0, 27),
     xlab="set Code",ylab="Error", boxwex=0.75, cex.axis=0.3)

and I would like to draw a transparent rectangle all over the plot between 2 defined y-values: (-50 ) and (100)!

I tried using the function rect as follows after the previous script:

  rect(0,-50,27,100, col= 'tomato2', density=10)

but this does not give me a uniform colored rectangle with transparency!!

Could anybody please help me in that? I almost spend over 2 hours until now on this with no success.

Many thanks in advance!

Solution

density will result in cross hatching, which is not what you want. What you want is alpha blending.

Try

# arguments to rgb(r,g,b and alpha) should be between 0 and 1.0
# this will make transparent blue
rect(x0,y0,x1,y1, col= rgb(0,0,1.0,alpha=0.5))

Comments

[user20650]

Define the colour using hex. eg rect(0,-50,27,100 ,col= '#FF003322'). The first 6 numbers give the red colour and the last two the level of transparency (i think)