There are several bands with this name:

1. The Brighton based seven-piece progressive-jazz-rock band. Woodwinds, Guitars, Basses, Drums, Singings, Synths, Pianos & Organs meld together to make the musical cake of Diagonal.
More information:
https://diagonal.bandcamp.com/
https://www.facebook.com/diagonalband

2. "Diagonal is a psychedelic six-piece from Chicago. Their new self-titled album, out now via Little Cloud Records, is a mix of highly polished psych rock, contemplative moments, and angular krautrock jams."
More information:
https://diagonalchicago.bandcamp.com/

3. Diagonal (1999 - 2004) was a Brazilian band from São Paulo influenced mainly by names of the "post-punk/post-hc" as Gang of Four and Jawbox. The musical proposal of the four members of diagonal is clear and explores to the maximum the possibility to try new paths and textures.
More information:
http://www.submarinerecords.net/english.html Read more on Last.fm. User-contributed text is available under the Creative Commons By-SA License; additional terms may apply.