Sketch of FVG. Observations in filters i and j measured at three different time instances (right). By pairing flux values of co-occurring observations, we form points in the flux-flux plot that fall on a line (dashed). Vector g_ij (in brown) corresponds to the unobserved host-galaxy and defines a line g_ij ⋅ x that intersects the dashed line at x₀, the point of (unobserved) minimum AGN activity. The FVG method consists of finding the intersection of u ⋅ x and the dashed line; u (in green) is a vector that has the same direction (i.e., same colors) as g_ij and therefore also intersects the dashed line at x₀.