I would caution against removing the levels of the 'By' group variable since, stating the obvious, without them it's not clear which 'subset' each chart relates to.
But, if I understand correctly, you want to make the labels in the outline nodes shorter. If you feel you have to do this you can try this way:
NamesDefaultToHere(1);
dt = Open("$SAMPLE_DATA/Big Class.jmp");
// Variability chart with 'By' group
vc = dt << variability chart(Y(:Weight),X(:Age),By(:sex));
// Report layer
vcRep = vc << report;
// Get the levels of the 'By' group
levels = AssociativeArray(Column(dt, "sex") << getValues) << getKeys;
// Update (shorten) outline node titles on the report
for(g=1, g<=NItems(vcRep), g++,
vcRep[g][OutlineBox(1)] << setTitle(Char(levels[g]));
);
To understand how it works, take a look at 'Help > Scripting Guide' or 'Help > Scripting Index'.
I'm just curious as to what the 'display problems' are that result from leaving things alone, though