I'm aware of displaying more resolutions with the option key like in this answer: https://apple.stackexchange.com/a/149152 but after upgrading to 10.10 the resolution of 3840x(i think)1080 of the lg ultrawide is not available anymore. The display is connected via thunderbolt. It changed after the upgrade, before it was no problem. The max resolution available is 2560×1080.
Mac – Native resolution not available on lg ultrawide with mac mini (2012)
mac-miniresolution
Best Answer
It requires a kext swap to an older graphics driver. They're probably trying to phase out older mac mini models with the release of the new ones.
Duplicate: Lost ability to choose 3440x1440 resolution after Yosemite upgrade