The hill-shaped curve f of x equals x times three minus x on top, and its running total A, the area collected from zero up to x, climbing from zero to four and a half, below, sharing one x axis from zero to three. Two buttons switch the reading direction. In "area equals climb" mode two draggable handles a and b mark an interval: the area shaded under the curve between them always equals the amount the running total climbs between the same two points. In "slope equals height" mode one draggable point x carries a tangent line on the running total and a height marker on the curve: the tangent's slope always equals the curve's height, greatest at the hilltop x equals one and a half and zero at both ends.

Read it in two directions

The hill f(x) = x(3 − x), and its running total A(x) — the area collected from 0 up to x. One object, two readings.