This option allows you to examine interpolated points between your data points. If you have a linear regression or curve fit defined for your data, then interpolate will show you the fitted point, otherwise, interpolate will show you a simple linear fit between the data points closest to your cursor.
Selecting Interpolate from the Analyze menu brings up a floating box (helper object) which displays the coordinates of any selected point on the graph. Uncheck the box next to Show on Graph from the Options dialog to hide the helper object from the page. Choose Additional Object Options --> Reveal Hidden Objects from the Options menu to call it back up.
Points may be selected by moving the mouse along the graph or by
pressing
the left and right arrow keys on the keyboard.