The blue line in the graph is part of the xaxis.
Note. The definition of Riemann Sums will be given in the development that follows.
which is called a partition of [a,b].
If each f(x_{i}) > 0 then the area of the i^{th} rectangle is
and the sum of the areas of the rectangles is then:
More generally, we do not require that f(x_{i}) > 0 as we define

If you want to view some additional graphs illustrating Riemann Sums with different values of n and different choices of x_{i}'s, then make your choices from the following two groups of options:
Note that the Riemann sum when each x_{i} is the righthand endpoint of the subinterval [a_{i1}, a_{i}] is
when each x_{i} is the lefthand endpoint of the subinterval [a_{i1}, a_{i}] is
and when each x_{i} is the lefthand midpoint of the subinterval [a_{i1}, a_{i}] is