A WIG file (.wig) is a text file that defines either a feature or data track. It must have a .wig file extension for IGV to read it as a wiggle file. The WIG file format is described on the UCSC Genome Bioinformatics web site: http://genome.ucsc.edu/FAQ/FAQformat.
IGV does not currently support multiple track lines in a single WIG file
One-based index: Start and end positions (for "fixedStep" and "variableStep" formats) are identified using a one-based index. The end position is excluded. For example, setting start-end to 1-2 describes exactly one base, the first base in the sequence.
Display settings: To modify IGV's default display settings for the WIG file data, include a track line in the file.