Added an option to piechart to allow users to preserve the order
of the values provided so that the pie section colors are always
consistant.
Changed preserve order method to also prevent cutting values from
the pie chart.
Renamed option for preserveOrder to preserveValues as this more
accurately refects the behaviour.
Added a test example with preserve values enabled.
Changed where check for preserveValue is
Merge remote-tracking branch 'zopa/master' into general_graph_improvements