This is an example of a definition for a graduated symbol theme:
<ObjectThemeLayer
id="id5"
name="Graduated Symbol Theme"
alias="Graduated Symbol Theme id5">
<DataSourceRef ref="id4" />
<FeatureGraduatedSymbolTheme id="id7">
<Alignment>
<HorizontalAlignment>center</HorizontalAlignment>
<VerticalAlignment>center</VerticalAlignment>
</Alignment>
<SymbolBaseSize useScale="false">
<MapScale>1</MapScale>
<DataValueAtSize>23</DataValueAtSize>
<PositiveSymbol>
<PointStyle>
<FontSymbol
text="""
size="36"
size-unit="mapinfo:length pt"
stroke="rgb(255,0,0)"
stroke-opacity="1"
family="MapInfo 3.0 Compatible" />
</PointStyle>
</PositiveSymbol>
<SymbolGraduation>sqrt</SymbolGraduation>
</SymbolBaseSize>
<SymbolLayout />
<NumericValueExpression>
<AttributeName>Indust_Growth</AttributeName>
</NumericValueExpression>
</FeatureGraduatedSymbolTheme>
</ObjectThemeLayer>