I propose an additional mode for LegendScaleActor: SetModeToFocalPlane
Here I will attempt codify exactly how the axis can be clearly defined with respect to camera position, the normal for the focal plane the view up and an origin (world or custom) that is the (x,y,z) origin projected (along FP normal) onto the Focal plane with the view up as the legend grid vertical access, the FP normal as the Z axis, and the horizontal axis being (view up) x (FP normal), and from the perspective of the camera the scene would dynamically always be looking in the -Z direction. And because the legend grid is essentially linked to the focal plane, there may need to a corresponding toggle to reverse the horizontal axis by flipping the grid axis Z to be the negative FP normal. This is the equivalent of moving the camera to the opposite side (+Z/-Z view direction) relative to a fixed world space x,y,z axis, but because here there would be a dynamic on-the-fly x’,y’,z’ axis tethered to the camera, there would need to be a toggle, reversing the verticle direction could just be done with the rotating view up the same as with SetModeToCoordinates. This modes behavior would match that of SetModeToCoordinates when the camera exists along a main world axis (in the positive direction) and is positioned in a plane orthagonal to the same axis and pointing along said axis towards the origin (except it is always pointing in the -Z direction)
In the video below I transformed the actors to simulate the scene as if the LegendScaleAnnotation functioned as I propose with the camera facing the same direction as Z (+Z view direction) as derived from the focal plane normal, the default behavior would be -Z view direction; therefore, SetModeToFocalPlane would require an assotiated toggle to flip this direction.
