Velocity obtained from each fitted line, for each inspected spectrum, with respect to the Chandra satellite reference frame. The red and blue diamonds indicate lines included in the cool and hot line sample, respectively, and errors refer to the 1σ level. The red and blue dashed lines correspond to the average velocities, v_Xc and v_Xh, obtained for the cool and hot line sample. The velocity measured for the Fe xvii line at 16.78 Å, not included in the average velocity computation, is indicated with an open diamond. To clarify, we included a small wavelength offset in those points corresponding to the same line fitted in the two gratings.