How to retrieve data points from chart image?
there is a little and simple software develop here in Brazil that can do this kind of thing.
You have to download in the following link: http://paginapessoal.utfpr.edu.br/lasouza/analise-nao-linear-de-estruturas/Pega%20Ponto%201.0.exe/view
You can load the image, specify the origin and set the x and y labels. After you can retrieve the points that you clicked on the image.