Your fun is a function of two variables, but integral is for functions of one variable.
Your fun is ignoring both inputs and returning a constant value.
Your br2 and brend would have to be numeric floating point values to use in the way you are. class(br2) and class(brend) would need to be 'single' or 'double'.
You are computing a single chi for each iteration, and using it as the x value for plotting Y, which is of unknown size. You might not get any output. You should probably be recording the chi results into a vector and plotting them afterwards, such as
chi(n) = integral(....);
and then after the loop,
plot(chi)