(I think this might require JMP17 and hopefully this gets better in JMP18) Use Properties button to open properties for your graph, select axisbox and modify the width
This does seem to be a bit buggy though, what I did was to disable stretching, set the line width to 20, press enter and re-enable stretching. JMP is able to capture this as a script BUT I would consider that it hasn't been yet implemented properly in JMP17.2 as there is no proper documentation in Scripting Index
Graph Builder(
Size(541, 908),
Show Control Panel(0),
Variables(X(:weight), Y(:height)),
Elements(Points(X, Y, Legend(3)), Smoother(X, Y, Legend(4))),
SendToReport(
Dispatch({}, "weight", ScaleBox, {Label Row(Line Width(20))}),
Dispatch({}, "height", ScaleBox, {Label Row(Line Width(50))})
)
)
Other options could be reference line OR using graphic script
-Jarmo