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I use pgfplots to draw a black-white array resembling imshow() in MATLAB.

I use the following code (from Plot a matrix of real values with TikZ/PGF like imshow in matlab ) to do it

    \begin{axis}[x=4mm,y=4mm,xtick=\empty,ytick=\empty,colormap={bw}{gray(0cm)=(0); gray(1cm)=(1)}]
    \addplot[scatter,scatter src=explicit,
             only marks, mark=square*,mark size=2mm]
             table[x=x,y=y,meta=l] {datafile.txt};

But if the blocks are all black, the boundary line between them disappears. I want the boundary line to be more apparent. How can I do that?

I have problem posing image, see

enter image description here

for an example.

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closed as too localized by percusse, Jake, Joseph Wright Jun 12 '12 at 21:05

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Could you include an example of what your datafile looks like? –  Jake May 12 '12 at 8:38
Do you want the line to be white, or gray, or some other colour? –  Jake May 21 '12 at 9:01
@Jake Do you think it's time we close this one? –  percusse Jun 12 '12 at 15:45
@percusse: Yes, I think so. There'd be too much speculation required for an answer, and if anyone needs this later on, they can just ask a new question. –  Jake Jun 12 '12 at 15:55
@Jake Voted as TL. –  percusse Jun 12 '12 at 15:59

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