Since we are converting a two-dimensional data array into a one-dimensional plot, the data have to be collapsed in some way. The possible variables are Q (the magnitude of the momentum transfer), Q2 (the momentum transfer squared), 2-theta (the scattering angle, in degrees), Chi (the azimuthal angle, in degrees), Qx (the horizontal component of the momentum transfer), Qy (the vertical component of the momentum transfer), Ellipse (used for azimuthal plots, the integration is done going around an elliptical rather than circular path), or the fraction of the distance along an arbitrarily chosen line segment. A drop-down menu allows you to choose which will be the plotting parameter and which will be the parameter over which we sum or average. For example, selecting "Plot versus Q / Sum or average over Chi" causes the data to be plotted versus Q (a radial plot) while integrating over the azimuthal angle. Selecting "Plot versus Q^2 / Sum or average over Chi" together with a logarithmic y axis (described below) produces a Guinier plot of log(intensity) versus Q^2. Selecting "Plot versus Chi / Sum or average over Q" gives a polar plot of intensity versus azimuth at constant radius, while selecting "Plot versus Chi / Sum or average over Q-Ellipse" produces a plot of the azimuthal intensity traced along an elliptical path. (Normally you probably want a circular path; the elliptical path might be useful in cases of a distorted scattering pattern resulting from strained samples). Note that the scales of Q and 2-theta depend on having first properly calibrated both the center and the scale of the image, using for example the Calibrate panel.
For most variables, when the plot is made the independent variable will be the actual value of Q, Chi, or whatever. For Line plots, there doesn't seem to be an obvious choice for what we mean by the independent variable, so the intensity is plotted versus fraction of distance along the chosen line segment (0 to 1).