Integral of Y with respect to X or area under the Y curve.
Note
Trapezoid rule is not the most accurate way of calculating integrals (it is
exact for linear functions), for example Simpson's rule (exact for linear and
quadratic functions) is more accurate.
# integral of sine function in [0, pi] range suppose to be exactly 2.
# lets calculate it using 10 samples:
x = (1:10)*pi/10
trapz(x, sin(x))
# now lets calculate it using 1000 samples:
x = (1:1000)*pi/1000
trapz(x, sin(x))