If there is no color difference between the graph line and the axes and grid lines (they are all black lines), using the Image | Vectorize Image command is probably not going to work very well.  It would only work well if the graph line was not surrounded by grid/axis lines of the same color, or if the graph line was a different color than the background grid and axis lines.  Currently, there is no way Didger 5 can easily pick out only the graph line and ignore all the same-color grid and axis lines surrounding it (the horizontal and vertical lines). To add a vote for this feature, email didgersupport@goldensoftware.com.

Therefore, the fastest way to digitize the graph line is to either:

1. Use the Map | Graticule and Grid command to add thicker, different colored grid lines over top of the existing image grid lines. Then you can use Image | Vectorize Image to automatically digitize the contour color.
2. Use the Draw | Polyline command and digitize them manually. In some cases, this will still be the best way to digitize information, and this is one of those cases.  If you use Image | Vectorize Image, you are going to have to spend more time editing the results than if you digitized by hand in the first place.
3. Use a third party program (like Adobe Photoshop) to erase the grid lines and all extraneous lines from the graph.  Then import it into Didger and use the Image | Vectorize Image command, choosing the "Complete Image" option. 
4. Use a third party program (like Adobe Photoshop) to change the graph line only to a distinctly different color from the grid lines.  Then import it into Didger and use the Image | Vectorize Image command, choosing the "Selected Colors" option.

Updated May 22, 2018