HP 48G Series Quick Start Manual page 86

Step 4: Optional. Shade the area between the current function and
the z-axis, and between the z-values represented by the
mark and the cursor. Note that the shading is for display
purposes only and is entirely optional to the process of
finding the area under a curve.
[200M[3.72 [TRACE] FEM EDITJCANCL
6-18 Plotting Equations and Analyzing Graphs