I have replicated your findings. Please contact JMP support support@jmp.com and submit the bug to them.
Here is a work around that does correct the problem.
Graph Builder(
Size( 661, 556 ),
Show Control Panel( 0 ),
Variables( X( :Column 1 ), Y( :"Column 4 [g/L]"n ) ),
Elements( Line( X, Y, Legend( 7 ) ), Points( X, Y, Legend( 8 ) ) )/*,
SendToReport(
Dispatch( {}, "Column 4 [g", TextEditBox, {Set Text( "Test [g/L]" )} )
)*/
);
current report()["Graph Builder",TextEditBox(4)] << Set Text( "Test [g/L]" );
Jim