Description of the desired feature

Originally posted in the forum.

For polar projections, there is no simple way to draw lines along the theta or r direction. It would be good to add -Ar|t to draw lines (1) -Ar: following the r direction then theta direction; (2) -At: following the theta direction then r direction.

A minimal example:

gmt begin map
gmt basemap -R0/60/0/10 -JP10c -Baf
gmt plot -W2p,red -At << EOF
0  5
60 5
EOF
gmt end show

Expected output: