L = 3;
a = 60;
b = 200;
c = 40;
d = 220;
z = 255;

x  = [0 a b z];
y1 = [c c d d];

figure(1)
plot(x,y1, 'LineWidth', L);
axis([0 255 0 255]);
axis square;
set(gca, 'xTick', [a b]);
set(gca, 'xTickLabel', {'a', 'b'});
set(gca, 'yTick', [c d]);
set(gca, 'yTickLabel', {'c', 'd'});


x = [a:b];
y = ( (x - a)/(b-a) ).^0.4*(d - c) + c;
x = [0 x z];
y = [c y d];

figure(2)
plot(x,y, 'LineWidth', L);
axis([0 255 0 255]);
axis square;
set(gca, 'xTick', [a b]);
set(gca, 'xTickLabel', {'a', 'b'});
set(gca, 'yTick', [c d]);
set(gca, 'yTickLabel', {'c', 'd'});


x = [a:b];
y = ( (x - a)/(b-a) ).^1.6*(d - c) + c;
x = [0 x z];
y = [c y d];

figure(3)
plot(x,y, 'LineWidth', L);
axis([0 255 0 255]);
axis square;
set(gca, 'xTick', [a b]);
set(gca, 'xTickLabel', {'a', 'b'});
set(gca, 'yTick', [c d]);
set(gca, 'yTickLabel', {'c', 'd'});