在R，Mac OSX El Captian中安装“rgl”软件包

I'm new to programming in R so I apologise in advance if I seem naive. I want to be able to do some principle components analysis on my data in 3D. I read that the "pca3d" function is good for this, but I need to download the "rgl" library. When I use install.packages it asks:I'm new to programming in R so I apologise in a