\documentclass[10pt,a4paper]{article}
\usepackage{pgfplots}
\pgfplotsset{compat=newest}
%\pgfset{declare function={f(\x)=3*exp(-(\x))*(\x)^3+1;}}
\pgfset{declare function={f(\x)=sin(2*deg(\x))*exp(0.1*\x)+2;}}
\pgfplotsset{
integral segments/.code={\pgfmathsetmacro\integralsegments{#1}},
integral segments=3,
integral samples/.code={\edef\integralsamples{#1}},
integral samples = 5,
integral min/.style args={#1:#2}{
ybar interval,
domain=#1:#2,
samples=\integralsegments+1,