gmt begin line_offset
gmt math -T10/30/1 T 20 SUB 10 DIV 2 POW 41.5 ADD = line.txt

gmt basemap -R8/32/40/44 -JM5i -Bxaf -Bya2f1 -BWSne --MAP_FRAME_TYPE=plain
gmt plot line.txt -W0.5p,red
gmt plot line.txt -W2p+o1c/500k
gmt text -F+f10p+jCM+a << EOF
11.0 42.6 -11.5 1 cm
27.1 42.3 +9.5 500 km
EOF
rm line.txt
gmt end show