I know you can set up grid lines on scatter plots but those are rectilinear. For something like a G/G plot it would be more useful to have contour lines (like on an Ordnance Survey relief map [atlas type map]) centered on 0,0 that would show constant radius "grid" from the 0,0 centroid. Think: make a plot like a the G/G ball widget from the video overlay. You could generalize this so that properties would allow the center could be offset and contour intervals could be varied; both with fairly simple value entry boxes in the dialog.
The reason this comes up is that it's actually pretty imprecise to set up a scatter plot so the x/y scale ratios are "square" so that the plot doesn't look artificially oblong which in turn makes it more visually tricky to see, in the case of the G/G plot for instance, when you are riding the edge of the circle (ahem, oval?? egg??).
Is this possible already and I missed it or is it something that could be added?? I know someone else asked for JPG backdrops for scatter plots...I guess that might be useful for this aside from the fact that getting the axis aspect ratios would still be a bit by-eye.