define function to data set

Hello, please if you could help me to fit a function to a data series, I understand that the 'fit' function of matlab works excellent, but it does not give me good results. What I have to do is fit the best possible function to a figure that has the semi-logarithmic vertical scale. I attach the data and the respective figure. Thanks greetings.

load x.txt
load y.txt
semilogy(x,y,'ob')

Answer 1

[KSSV]

x = importdata('x.txt') ; 
y = importdata('y.txt') ; 

y = log(y) ; 

p = polyfit(x,y,2) ; 
yi = polyval(p,x) ; 

plot(x,y,'.r',x,yi,'.b')

[ignacio bobadilla tapia]

How can I make the equation display on the graph?