We discuss the phase properties of a double-grating compressor with grazing-incidence gratings in the off-plane mount, designed for the temporal compression of XUV attosecond pulses produced with the technique of high-order harmonic generation. Its purpose is to introduce a negative chirp that compensates for the intrinsic chirp of the pulse. The study is based on the path lengths of the rays at different wavelengths, and their control in order to achieve either positive or negative group-delay dispersion. We demonstrate that the sign and the amount of the dispersion introduced is controlled by a linear translation of a grating. Beside the instrument is expected to present a high throughput, constant along the spectrum of interest. The compressor can be designed for any spectral region in the XUV and soft X-ray domain. As a test case, the applications to the compression of attosecond pulses centered at 70 eV and at 160 eV are discussed.