The integral function calculates the area under a segment of the graph. The selected area is filled on the graph and a helper object displays the numeric result. The integral is calculated using the trapezoid rule.
Click and drag the mouse across the segment that you want to integrate. The shaded area marks the beginning and end of the range. If you don't select a range, the curve will be calculated to the entire range of data displayed in the graph.
Choose the integral function either by clicking the Integral
button 
on the toolbar or by
selecting the Integral option from the Analyze
menu. You can click and drag the brackets and the integral
will update automatically. To remove the integral from the graph, click
the close box in the upper corner of the integral helper object. To
edit the integral's properties, double-click the helper object. The Integral Options
dialog box will appear.