Visualization of the results of a loop of spectral fits onto the time-averaged XMM-Newton / RGS spectrum of the Ultraluminous X-ray source NGC 247 ULX-1. The parameter space explored by the spectral fits covers a range of wind velocities (as constrained by the Doppler shift of the absorption lines) and photoionization parameters 𝜉 (an indicator of the wind temperature measured through the relative line ratios). The color code of the 2D map indicates, for each individual spectral fit, the improvement provided by the wind plasma component on the broadband continuum model. The black contours show the (2.0, 2.5, … 5.0 sigma) confidence levels, suggesting that the wind blows at about 17% of the speed of light. These results provide one of the most convincing detections of a hot plasma outflow in ultraluminous X-ray sources, the very first in the subclass of the supersoft sources. The implications of this discovery are discussed in the contribution of C. Pinto and P. Kosec, this issue, e20220134