Time series plot with decimal year input

Dear all,

I am new to GMT but before posting this question I did my homework: watched all unavco videos and worked through some useful for me examples. I would like to plot velocity time series of three gps sites (sit1, sit2, sit3 attached): decimal year (the first column) against velocity in mm/hour (the fourth column). This is the furthest I could get:

gmt begin ts_vel_arg1 pdf,png
gmt set FORMAT_DATE_IN dd-o-yy
gmt plot vel_sit1_206h.out -R2020.000/2020.999/0/15 -JX20c/10c
-i0,3 -Sc0.2s -Gred -B
#-i0,3 -W2p,red -B
#-Bpxa7Rf1d -Bsxa1O
gmt end show

I could not find set FORMAT_DATE_IN decimal year in the documentation. Ideally, I would like to plot all three stations at the same plot connecting all my dots and make it prettier with labels and legend. Any help or hint would be appreciated!

Thank you for your attention,
vel_sit1_206h.txt (43.7 KB) vel_sit2_206h.txt (28.0 KB) vel_sit3_206h.txt (32.4 KB)

FORMAT_DATE_IN etc has to do with proper absolute time records. Your decimal years are, I believe, a bit fake since years have different lengths so 2020.xxxxxx is not necessarily the same date as 2021.xxxxxx. You should just treat your decimal years as Cartesian coordinates.
If you want real, fancy abs time annotations (month names, etc) you will have to plot the baseframe separately with basemap and give actual ISO timestamp start/stop etc (e.t., 2020-01-1T/2020-09-15T14:35), then overlay your Cartesian data with its own near-matching -R and same linear scaling - you probably cannot tell the difference.

Thank you very much Professor Wessel for your detailed response! I will try to implement your comments.