t=[0 0.92 1.84 2.95 3.87 4.98 5.9 7.01 7.93 8.97 10.95 12.03 12.95 13.88 14.98 15.90 16.83 17.93 19.04 19.9 20.84 23.88 24.99 25.91];
h=[968 948 931 913 898 881 869 852 839 822 1082 1050 1021 994 965 941 918 892 866 843 822 1059 1035 1018];
c2=polyfit(t(11:21),h(11:21),4);
a2=polyder(c2);
dt3=diff(t(22:24));
dh3=diff(h(22:24));
dht3=-dh3./dt3;
t3=[20 20.8 t(22) t(23)]
xx3=[-polyval(a2,t3(1:2)),dht3];
c3=[-polyfit(t3,xx3,3)];
a3=polyder(c3)
tp3=20.8:0.1:24;
x3=-polyval(c3,tp3);
plot(tp3,x3,'c.')
hold on 
axis([0 25 12  34])
xlabel('hour')
ylabel('cm/houor')