from screen to file

I use a program that produce a graphic (imagine a sinusoid, more or less).
There is no opportunity to obtain from the program an output of the coordinates of the points of this curve.
I desperately tried a way to achieve this, for example using a program that transform a normal raster image in a vector, and after read the vector, but all this is not precise nor accurate.
I wonder if there is a way to obtain this result.
Bye