function p = my_linear(x,y,t);

n = length(y);

for i=1:n-1,
  if  x(i) <= t & t <= x(i+1)
      p = y(i)*(t-x(i+1) )/(x(i)-x(i+1)) + y(i+1)*(t-x(i))/(x(i+1)-x(i));
      return
   end
end