I noticed this recently too, apparently in any of the scrolling modes the background is always opaque. Kind of too bad, it would be nice to utilize it for a piano-roll kind of display, where there were keys underneath. Don't know if it can/will be changed, maybe it has to be opaque for some reason.
A hack workaround could use [itable] for the data, constantly shifting your data as you get new values in, but obviously that's more of a pain. Though I couldn't resist trying.... this is kind of fun :)