An object that supplies Font instances associated with
keys. This is used by the PiePlot3D class to obtain section label
fonts for each data item(segment) in the chart. A default implementation
(StandardFontSource) is provided.
Restyles the source using the specified font. Refer to the implementing
class to confirm the precise behaviour (typically all existing font
settings are cleared and this font is installed as the new default
section label font).