documentclass[tikz]{standalone}
usepackage[utf8]{inputenc}
usepackage{libertine}
usepackage[libertine]{newtxmath}   % Math fonts

usepackage{filecontents}
begin{filecontents*}{testt.txt}
-1 -1
1 2
2 4
3 2
4 1
5 0
end{filecontents*}

begin{filecontents*}{testp.txt}
-1 6
1 4
2 2
3 1.5
4 2
5 3
end{filecontents*}

begin{document}
begin{tikzpicture}[scale=2]
fill[gray!10] (1,1) rectangle (4,4);
node[rotate=90,above,font=small] at (1,2.5) {intensity};
node[below,font=small] at (2.5,1) {size};
begin{scope}
clip (1,1) rectangle (4,4);
% grid
foreach i in {1,2,...,4}
    {
    draw[gray!50] (i,1) -- (i,4);
    draw[gray!50] (1,i) -- (4,i);
    }
% axis labels
foreach i in {2,3}
    {
    node[gray!50,above,fill=gray!10,font=tiny,inner sep=2pt,yshift=2pt] at (i,1) {i};
    node[gray!50,below,fill=gray!10,font=tiny,inner sep=2pt,yshift=-2pt] at (i,4) {i};
    node[gray!50,below,fill=gray!10,font=tiny,inner sep=2pt,xshift=2pt,rotate=90] at (1,i) {i};
    node[gray!50,above,fill=gray!10,font=tiny,inner sep=2pt,xshift=-2pt,rotate=90] at (4,i) {i};
    }
% corner labels
node[gray!50,above left,fill=gray!10,font=tiny,inner sep=2pt,xshift=-2pt,rotate=90] at (4,4) {4};
node[gray!50,below right,fill=gray!10,font=tiny,inner sep=2pt,yshift=-2pt] at (1,4) {1};
node[gray!50,above left,fill=gray!10,font=tiny,inner sep=2pt,yshift=2pt] at (4,1) {4};
node[gray!50,below right,fill=gray!10,font=tiny,inner sep=2pt,xshift=2pt,rotate=90] at (1,1) {1};
draw[line width=1pt] plot[smooth] file {testt.txt};
draw[line width=1pt,red] plot[smooth] file {testp.txt};
end{scope}
draw[color=gray] (1,1) rectangle (4,4);
end{tikzpicture}
end{document}