D = [ 3.85 3.80 5.50 7.38
      3.75 4.00 6.75 8.51
      3.70 4.30 7.25 9.52
      3.70 3.70 5.50 7.50
      3.60 3.85 7.00 9.33
      3.60 3.80 6.50 8.28
      3.60 3.75 6.75 8.75
      3.80 3.85 5.25 7.87
      3.80 3.65 5.25 7.10
      3.85 4.00 6.00 8.00
      3.90 4.10 6.50 7.89
      3.90 4.00 6.25 8.15
      3.70 4.10 7.00 9.10
      3.75 4.20 6.90 8.86
      3.75 4.10 6.80 8.90
      3.80 4.10 6.80 8.87
      3.70 4.20 7.10 9.26
      3.80 4.30 7.00 9.00
      3.70 4.10 6.80 8.75
      3.80 3.75 6.50 7.95
      3.80 3.75 6.25 7.65
      3.75 3.65 6.00 7.27
      3.70 3.90 6.50 8.00
      3.55 3.65 7.00 8.50
      3.60 4.10 6.80 8.75
      3.65 4.25 6.80 9.21
      3.70 3.65 6.50 8.27
      3.75 3.75 5.75 7.67
      3.80 3.85 5.80 7.93
      3.70 4.25 6.80 9.26
      ];
y = D(:,4);
x1 = D(:,2)-D(:,1);
x2 = D(:,3);
n = size(D,1);
%---------
close; figure('Position',[100 100 900 400]);
subplot(1,2,1); hold on;
plot(x1,y,'ro','markersize',10,'markerface','c');
p1 = polyfit(x1,y,1);
xx1 = linspace(-0.2,0.6,100);
yy1 = polyval(p1,xx1);
plot(xx1,yy1,'r-','linewidth',3);
xlabel('x_1','fontsize',18);
ylabel('y','fontsize',18);
%---------
subplot(1,2,2); hold on;
plot(x2,y,'bo','markersize',10,'markerface','y');
p2 = polyfit(x2,y,2);
xx2 = linspace(5,7.5,100);
yy2 = polyval(p2,xx2);
plot(xx2,yy2,'b-','linewidth',3);
xlabel('x_2','fontsize',18);
ylabel('y','fontsize',18);
%---------
alpha = 0.05;
x     = [ ones(n,1) x1 x2 x2.^2];
[b,bint,r,rint,stats] = regress(y,x,alpha)