What is plotted vs what is used as a label can be different. I suggest plotting the negative of KPI(2) and Parameter2, but label the axis using positive values.
KPI1 = [0 0.125 0.25 0.5];
P1 = [0 0.5 1 2];
KPI2 = [0 0.07 0.125 0.375];
P2 = [0 0.5 1 2];
plot(P1,KPI1)
hold on
plot(P1,-KPI2)
plot(-P2,KPI1)
plot(-P2,-KPI2)
hold on
% Define axes ticks and labels
xticks([-3 -2 -1 0 1 2 3])
xticklabels([3 2 1 0 1 2 3])
yticks([-0.5 -0.25 0 0.25 0.5])
yticklabels([0.5 0.25 0 0.25 0.5])
axis([-3 3 -.75 .75])
% place axes at origin
ax = gca;
ax.XAxisLocation = 'origin';
ax.YAxisLocation = 'origin';
