data=[0 1.00
1.38 0.95
1.48 0.93
1.58 0.93
1.68 0.91
1.78 0.90
1.88 0.89
1.98 0.84
2.08 0.81
2.18 0.77
2.23 0.75
2.33 0.74
2.43 0.69
2.53 0.65
2.63 0.62
2.73 0.60
2.83 0.56
2.93 0.54
3.03 0.51
3.13 0.48];
x=data(:,1);
y=data(:,2);
close  all
% figure
% plot(x,y);
% grid on

% cftool ;
% https://jingyan.baidu.com/article/e5c39bf5c86ed039d760331c.html

syms t
% f=fittype('A2+(A1_A2)/(1+exp((t-x0)/dx))','independent','t','coefficients',{'A1_A2','A2','x0','dx'});
f=fittype('A2+(A1-A2)/(1+exp((t-x0)/dx))','independent','t','coefficients',{'A1','A2','x0','dx'});
cfun=fit(x,y,f); %显示拟合函数，数据必须为列向量形式
xi=0:0.01:7;
yi=cfun(xi);
figure
plot(x,y,'r*',xi,yi,'b-');
title('拟合函数图形');
grid on
% figure
% plot(x,sqrt(y-yi(1:1:18,:).^2/18),'r*');
% title('标准误差图');

matlab fit函数拟合实例