I have found the solution.
Linear interpolation is ok for this purpose.
data= [0 0;
1 1;
2 2;
3 2;
4 1;
5 1.25;
6 3.25];
figure
plot(data(:,1),data(:,2),'r+');
hold on
xq= min(data):0.1:max(data);
yq = interp1(data(:,1),data(:,2),xq);
plot(xq,yq,'k o');
