There is currently no way to create a 2D scatterplot for a scroll image, unfortunately. You could attempt to do so with a custom program written in IDL, but the major limiting factor is system memory. If your image is large, you might exceed the maximum amount of available RAM while attempting to grab data from two entire bands and plot the values against each other. You can get a larger area by setting your screen resolution as high as possible and then maximize the Image window, but that still might not cover your entire image.

How about now? For now Is there anyway to create a 2D scatter plot for the whole image?

It is only possible for relatively small images in ENVI 5. The Help system explains: "Select the Viewable Area Only box to limit the scatter plot computations to the visible part of the bands. This option is useful for large files because of the excessive memory required for the calculations. This option is selected by default. For very large files, this option is selected and locked. The scatter plot uses the displayed pyramid level. For example, if a large data set is displayed and the magnification is reduced to fit the entire data set on the screen, the scatter plot is based on the pyramid level displayed rather than all the pixels of the selected bands." - Peg Exelis VIS

Thanks for your reply Peg. Actually I tend to use ENVI Classic instead of ENVI 5. If I want to adjust the small red box in the scroll window (the content in the red box is shown in the image window), how do I do that? The size setting in the preference seems has nothing to do with the red box.

You drag the corner of the main image window to make it larger. Then the red box will become larger to match the new extent of the image window. - Peg Exelis VIS

I see. Thanks a lot!