I have been reading about the deviation of galactic rotation curves from the predicted Keplerian curve to a flat curve at large galactocentric distances.

They were the first indicators of the existence of dark matter but I do not understand how the curve is obtained from Doppler shift data (as mentioned by some papers as the method of determination for the curve.)

Doppler shift data would allow measurement of radial velocity but how can that alone be enough to extrapolate the rotation curve?