changing the color of missing data in 2d plot

[Etienne]

I would like to change the color of missing data from
the background color (white in my case) to another
color (grey).

If I use set background 15 the missing data is grey
but the whole image (plot area and all that
surrounding) is also grey.  So how can I

1) change the "missing color"

or

2)change the background color only for the plot area
(not everything surrounding it).

Thanks,
Etienne


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