You can import graphs by pasting it from the clipboard (Edit->Paste Graph) . This can be used to import graphs in formats, that are not supported by DigitizeIt e.g. pdf-documents. Just open the graph with any image editing application, copy it to the clipboard and paste it into DigitizeIt.
Tip: The fastest way to import graphs in unsupported formats is by using DigitizeIt's capture screenshot feature.