Make scale bar say "0 0.5 1 1.5" instead of "0 0.75 1.5"

[jidanni]

Here we see a scale bar with “0 0.75 1.5”,
made with gmt basemap -LjTR+w1.5M+ab+l+f

What if I want it to instead say “0 0.5 1 1.5”?

[pwessel]

Sorry, it is an automatic selection.