Extract data from an imported plot

Question

I have a plot of an unknown function stored as a PNG file in a folder in which Mathematica file also is saved. I have used

SetDirectory[NotebookDirectory[]]; 
importedplot = Import["plot.png"];

I am going to redraw it by ListPlot if I was able to extract data from this plot.

I have seen this page and this one but I could not understand what must be replaced by for example Plot[Tan@x, {x, 0, 2 Pi}] in command

 data = Cases[Plot[Tan@x, {x, 0, 2 Pi}], Line[data_] :> data, -4, 1][[1]];
Export["file.txt", data, "Table"]

As it used in the pages addressed. Can anyone guide me!