Extract data from chart

Hi,

Usually, data are given by a chart/figure in picture form, such as JPG,TIF,BMP,PNG...

Like below, it is the V-I charactristic of a diode.

I want to get the V and corresponding I value, then I can get a fit function V(I) (or I(V) ).

Although,we can read several more exact points by eye-measurement, and then get a fit function. I think the fit function will be more precise if we read more points.

How can I do to extract as much points as possible? Any help will be appriciated.

My MC version is "Mathcad 15 F000".