If you have plotted a function, then you must have the function definition in the first place. so you can just use the Table command to generate the data. You can also specify the origin using AxesOrigin option. If you really want to get the data from the plot, it depends on the plot type. For Plot you can try

   p = Plot[Sin[x], {x, -Pi, Pi}]
   data = (InputForm@p)[[1, 1, 1, 3, 2, 1]]

   {{-3.14159, -1.28228*10^-7}, {-3.13967, -0.00192716}, {-3.13774, -0.00385419},......

For ContourPlot3D I use data = (InputForm@p)[[1, 1, 1]] There might be better ways, but this is what I know.