This plugin creates a scatter plot of two bands of a raster (or two raster files with the exact same shape and location) then you can select areas on the scatter plot and export it as raster or vector.
This tool is present in ERDAS and ENVI but there was no such tool in QGIS. To install: Go to Plugins tab -> manage and install plugins then search for feature space.
After the installation is complete a new tool will be added under Raster tab