Does the graph you have state what the optical power is for each wavelength, or is it for constant lumens? And is the output (y-axis) in current (amperes) or voltage? The first answer to this question is correct, you need to be sure the incident light is quantified in terms of radiant power (in watts).
You need to know the spectral irradiance which is the incident irradiation power per wavelength interval at the different wavelength of the measurement range.
Assuming this power is P(lambda) watt/ um ,
Then you obtain the responsivity by dividing the total current to the total power.
The total current can be obtained by integrating the spectral current curve which you got and the total irradiance= Integral P(lambda) dlmbda across the whole measuring band.