f(x)equals to 2x is in form of y equals to ax or y equals to af(x) (if f(x)equals to x) plot f(x) equals to 2x
f(x) = 2x
Looks like y= ax, with a=2
or
y = 2f(x) if f(x) = x
not quite sure just what it is you're asking.
enter your expression at wolframalpha.com and see the graph.