TikZ is used for reproducing a diagram by Lê Nguyên Hoang seen: on http://www.science4all.org/le-nguyen-hoang/integer-programming/
The code was written by Henri Menke and published on TeX.SE.

Edit and compile if you like:
% Coloring diagrams - linear relaxation% Author: Henri Menke\documentclass[tikz, border=10pt]{standalone}\usetikzlibrary{shapes}\begin{document}\begin{tikzpicture}[every path/.style = {},every node/.append style = {font=\sffamily}]\begin{scope}\shade[right color=green, left color=white, opacity=0.7](-0.5,-0.5) rectangle (0,6.5);\node[rotate=90, above] at (0,3) {Non-Negative Gold};\shade[top color=yellow, bottom color=white, opacity=0.7](-0.5,-0.5) rectangle (8.5,0);\node[below] at (4,0) {Non-Negative Bills};\shade[left color=red, bottom color=red, right color=white, opacity=0.5](-0.5,5.5) -- (8.5,3) -- (8.5,6.5) -- (-0.5,6.5) -- cycle;\path (-0.5,5.5) -- node[pos=0.23, sloped, above] {Volume Constraint}(8.5,3);\shade[left color=blue, right color=white, opacity=0.5](2.5,6.5) -- (8.5,6.5) -- (8.5,0) -- (5,0) -- cycle;\path (5,0) -- node[pos=0.3, sloped, above] {Weight Constraint} (2.5,6.5);\node[text width=7em, align=center] at (2,2){Feasible Region of the Linear Relaxation};\draw[->] (-0.5,0) -- (8.5,0) node[below] {Gold bars};\draw[->] (0,-0.5) -- (0,6.5) node[above] {Bill stacks};\node[rotate=-45, above, text width=9em, align=center] at (7.25,5.25){Level Curves of the Objective Function};\path[clip] (-0.5,-0.5) rectangle (8.5,6.5);\foreach \i in {0.5,3,...,13} {\draw[help lines] (-0.5,\i) -- +(-45:15);}\end{scope}\draw[very thick, ->] (9,3.25) -- node[above, text width=4cm, align=center]{\Large\bfseries Add Integrality Constraints} (13,3.25);\begin{scope}[shift={(14,0)}]\shade[right color=green, left color=white, opacity=0.7](-0.5,-0.5) rectangle (0,6.5);\shade[top color=yellow, bottom color=white, opacity=0.7](-0.5,-0.5) rectangle (8.5,0);
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