explicitly specifying --MAP_ANNOT_OBLIQUE=anywhere seems solving the map frame annotation placement issue in gmt 6.5.0:
gmt basemap -JOb168/51/172/32.5/14c -R-500/2300/-400/400+uk -BeNsW -Bxa5f1 -Bya5f1 -png quick --MAP_FRAME_TYPE=plain --MAP_ANNOT_OBLIQUE=anywhere
the documentation says it should be there as a default gmt.conf — GMT 6.6.0 documentation but somehow this was not the case
@yvonnefroehlich
I did not understand the problem either until @Joaquim posted the frames
