Try this script against the fitness data. I still don't know if it's possible to change the font of a grouping variable, but this code will change the right hand axis if it's used for a secondary Y axis.
Graph Builder(
Variables(
X( :Runtime ),
Y( :Oxy ),
Y( :Weight, Position( 1 ), Side( "Right" ) )
),
Elements(
Points( X, Y( 1 ), Legend( 1 ) ),
Smoother( X, Y( 1 ), Legend( 3 ) ),
Points( X, Y( 2 ), Legend( 6 ) ),
Smoother( X, Y( 2 ), Legend( 7 ) )
),
SendToReport(
Dispatch( {}, "Runtime", ScaleBox, {Tick Font( size( 16 ) )} ),
Dispatch( {}, "Oxy", ScaleBox, {Tick Font( size( 16 ) )} ),
Dispatch( {}, "Weight", ScaleBox, {Tick Font( size( 14 ) )} ),
Dispatch( {}, "graph title", TextEditBox, {Set Font Size( 16 )} ),
Dispatch( {}, "X title", TextEditBox, {Set Font Size( 16 )} ),
Dispatch( {}, "Y title", TextEditBox, {Set Font Size( 16 )} ),
Dispatch( {}, "Y r title", TextEditBox, {Set Font Size( 14 )} )
)
);