\begin{center}
\begin{tikzpicture}
\begin{axis}[
scale = 1.2,
xmin = 0, xmax = 10,
ymin = 0, ymax = 10,
axis lines* = left,
xtick = {0}, ytick = \empty,
clip = false,
xlabel={Quantity of energy efficient housing}, x label style={at=(current axis.right of origin), anchor=west},
ylabel={Price, costs and benefits (YEN)}, y label style={at=(current axis.above origin), anchor=south, rotate=-90},
]

% Colouring areas
\fill[green, opacity = 0.5] (5, 5) -- (5, 10) -- (7.5,7.5);
% MPB line
\addplot[color = blue, very thick] coordinates {(1, 9) (9, 1)};