Is plotlyjs a separate pkg or it is a backend in plots as gr?
Seriously though, it is a “separate” (as in standalone) package, but it can also be used as a backend in Plots.jl. The same is true for GR.
But before using we need to install all of separately or adding only plots is ok?
You need to install separately and use
plotlyjs(). Only GR is installed by default.