A bounded value is used to represent a datapoint
that a chart element intends to render on screen.
ChartElements report BoundedValues, describing their data
to an Axis object. The axis computes autogenerated ranges.
A BoundedValue encapsulates both the value of the datapoint
in a particualr dimension, as well as a margin, in pixels,
that a ChartElement needs above or below the value
in order to render it.
Plots on a plot chart, for example, report their data
with margins to accomodate the size of the plots.
A column chart might report a margin in the Y axis
to accomodate a label rendered above the chart.