I do not know of any official graph that has been made public (yet), but I’m going to guess that it will be extremely similar to any other board that uses the LMS7002M chip.
If you compare LimeSDR-USB output power vs frequency to the EVB7 development board output power vs frequency on page 37 (pdf warning). The values on both graphs are within a couple of dBm of each other.
EDIT: I found a graph that looks like the shape I would expect for the LimeSDR mini on an external website - the graph looks to be about 7dB lower than it could because the gains were not at maximum. But that is on the page if you interpret the presented information.