[R] Plot Rect Transparency

[Martin Maechler]

>>>>> Duncan Murdoch 
>>>>>     on Thu, 28 Jun 2018 20:57:19 -0400 writes:

    > On 28/06/2018 5:29 PM, Jeff Reichman wrote:
    >> R-Help
    >> 
    >> 
    >> 
    >> Is there a way to make a rectangle transparent (alpha=0.1??)
    >> 
    >> 
    >> 
    >> plot(c(100, 200), c(300, 450), type= "n", xlab = "", ylab = "")
    >> 
    >> rect(110, 300, 175, 350, density = 5, border = "red")
    >> 
    >> 
    >> 
    >> Can't figure out how to take the changepoint function results and plot in
    >> ggplot2 so I can just simply add rectangles to the plot function, but I need
    >> to make transparent and there doesn't seem to be an alpha option.

    > Alpha is part of the colour spec.  For example,

    > rect(110, 300, 175, 350, density = 5, border = rgb("red")


    > rect(110, 300, 175, 350, density = 5, border = rgb(red=1, green=0, 
    > blue=0, alpha=0.1))

    > I'm not sure what is the quickest way to work out the rgb values for a 
    > named colour (col2rgb can do it, but not in a convenient format) if you 
    > want to add alpha to it.

IIUC, it is adjustcolor() you were thinking of.  It had been
created to do that and more. 

I'm using that "all the time" nowadays in my graphics code,
e.g.,

> adjustcolor("red", 2/3)
[1] "#FF0000AA"