File g Load bitmap image
g from clipboard
g automatic (ctrl-shift-B)
This command loads a scanned graph bitmap image as a graph background into Spectrum viewer. After you have scaled the bitmap to the right range (see the command "Set bitmap range" below) you can use this to re-create the data from the bitmap by drawing/tracing it over the bitmap yourself (see this tutorial).
The "automatic" option looks if there's an image on the clipboard. If there is, it loads the image. If there isn't, it asks for a file to load. It should be the option of choice, except when there's an image on the clipboard and you explicitly want to load another image from a file.
You cannot load more than one bitmap at a time.
File g Clear bitmap image
Clears a loaded bitmap.
File g Set bitmap range (ctrl-shift-R)
When a bitmap is loaded it is automatically scaled to fill the graph as it is displayed at the moment of loading. This is usually not the scale you want the bitmap to be, so you can use this menu option to scale the bitmap to your liking.
First draw a rectangle on the screen (for example, over the axes of the graph bitmap), and then choose this command. You have to specify how large the rectangle should be and then Spectrum Viewer will scale the bitmap so that the rectangle would be the size you specified. You can then draw a new data set over it directly on the correct scale.
Above you can see the result of a bitmap of which the drawn rectangle was scaled to X 0-500 and Y 0-2000. Notice the difference in the values on the X-axis and the Y-axis.