Time axis phase

Q&A
1

Hi

(tested on GMT 5.4.3 and 6.4.0)

In plotting a time axis it seems that the tick/grid/label location always aligns with the first day of the month (for unit D of the stride). E.g. with -Bxa7D for the interval 2025-04-19 to 2025-05-03 I get tick/grid/labels at 2025-04-22, 2025-04-29, 2025-05-01 but would like to get them at 2025-04-19, 2025-05-26, 2025-05-03. Is there a way to achieve this? I tried to shift the tick/grid/labels by specifying a phase on the -Bx option but no avail, but it is a bit unclear to me how the phase should be specified for a time axis.

2

It doesn’t look like specifying phase works for time axis and there seems to be no examples in the docs. Custom annotations via -Bxc... work.

cat << EOF > xannot.txt                                                  
2025-04-19T a 2025-04-19
2025-04-26T a 2025-04-26
2025-05-03T a 2025-05-03
EOF
gmt basemap -JX10c/1c -R2025-04-19T/2025-05-03T/0/1 -Bxcxannot.txt -BwSne -png time_axis_phase.png

time_axis_phase
time_axis_phase1352×182 2.2 KB

3

Ok, that was my suspicion as well but I was hoping to be wrong. Thanks for your input.