  We report on the calibration of an argon/isobutane (97.7 %/2.3 %)-filled
GridPix detector with soft X-rays (277 eV to 8 keV) using the variable energy
X-ray source of the CAST Detector Lab at CERN. We study the linearity and
energy resolution of the detector using both the number of hit pixels and the
total measured charge as energy measures. For the latter, the energy resolution
{\sigma}_E/E is better than 10 % (20 %) for energies above 2 keV (0.5 keV).
Several characteristics of the recorded events are studied.
