Some thing to try are: Modifying the settings slightly in the Vectorize Image dialog box and using the image using the filters under Image | Processing Filters.

1. Using the Image | Processing Filters | Image Erosion and Dilation option (use the Dilation method and one pass to help thicken up the lines and eliminate any gaps).
2. Using one of the color filters to try and outline or enhance the lines (i.e. Convert to Grayscale, Convert to Black and White, Color Reduction, or Modify Image Colors under the main Image menu). For example, if you want to digitize only the blue lines in an image, go to Image | Processing Filters | Color Reduction and select the Popularity method and 4 colors. This will reduce the number of colors in the image to help Didger pick out only the color of interest (blue).
3. Adjusting the contrast or brightness might also help to remove minor extraneous lines and leave only the thick lines that Didger can easily vectorize.

4. In the Vectorize Image dialog, you can try increasing the Color Tolerance. If the image is saved in low quality (ie. JPG), sometimes the colors that may look solid are made up of many similar colored pixels. Increasing the Color Tolerance will help Didger ignore these slight color differences.

5. In the Vectorize Image dialog, you can try decreasing the Min Pixel Length. If Didger can't detect a long string of pixels with the specifications made in the Vectorize Image dialog, no polylines and polygons will be created. Decreasing the minimum pixel length may help since Didger can then vectorize small portions of polylines or polygons.

If there are a lot of grid lines or marks on the image that you can't remove with pre-processing the image (for example, a logarithmic graph with grid lines), then you can either erase the extra markings in another software (like Adobe Photoshop) or manually digitize the lines. With some images, manual digitizing will still be the best solution.

Updated: October 7, 2016