Hi Serena and Ansley,
Thank you for your help! You solved the mystery.
when the length of the vector is too small, ferret and pyferret plot a dot symbol, but pyferret 7.63 keeps the size that you have defined previously with the command: plot/vs/symbol/thick=3/size=0.5/ . . . . .
That was it. So, following your suggestion, I'll use this workaround
set data tmp.nc
let speed = (u_anom^2 + v_anom^2)^0.5
let uu = if speed gt 0.2 then u_anom
let vv = if speed gt 0.2 then v_anom
shade/nokey 0*u_anom ! dummy
plot/vs/symbol/thick=3/size=1/color=red/nolabel/ov\
{143.58333333333333333333}, {38.25}
!vector/aspect/ov u_anom, v_anom ! -> big dots!
vector/aspect/ov uu, vv ! -> Good
frame/file=tmp.png
(By the way, I've fixed my bug: My previous code plotted the red marker outside the plotting region.)
Best regards,
Ryo