Something like this:
x1=[0.8043 0];
y1=[0 0.0789];
x2=[0.8407 0];
y2=[0 0.0825];
x3=[0.8726 0];
y3=[0 0.0890];
x4=[0.1 0.2];
y4=[0 0.1];
% find intersections
m1 = (y1(2)-y1(1))/(x1(2)-x1(1));
m2 = (y2(2)-y2(1))/(x2(2)-x2(1));
m4 = (y4(2)-y4(1))/(x4(2)-x4(1));
xint14 = ((m1*x1(1)-m4*x4(1))-(y1(1)-y4(1)))/(m1-m4);
yint14 = m1*(xint14-x1(1))+y1(1);
xint24 = ((m2*x2(1)-m4*x4(1))-(y2(1)-y4(1)))/(m2-m4);
yint24 = m2*(xint24-x2(1))+y2(1);
hp = patch([xint14 xint24 x2(1) x1(1) xint14],[yint14 yint24 y2(1) y1(1) yint14],'k');
hp.FaceAlpha = 0.2;
hold on
plot(x1,y1,'k')
plot(x2,y2,'b')
plot(x3,y3,'g')
plot(x4,y4,'r')
xlim([0,0.9])
ylim([0,0.1])
xticks(xlim)
yticks(ylim)
xlabel('PVB content in binder')
ylabel('SA content in binder')
