[Thuban-list] RFC: Layer Classification

[Jonathan Coles]

RFC: Layer Classification
-------------------------

Thuban currently has no mechanism for classifying the shapes that
compose a layer. It should be possible to define classification
properties that partition the shapes so that each partition can be drawn
in a different way. For instance, shapes associated with counties can
have population properties which are then color coded. 

There should be a Classification class that encapsulates a
classification of a layer's shapes. It would contain the data type that
it was associated with (this would come from the table of shape
properties), and a list of data values:

[[range_low, range_high, [data_values, ...]], ...]

When a layer is drawn each shape matches its property value against the
ranges (range_low <= value <= range_high, however <= is defined) in the
list to find its assigned data values. These values can determine such
things as line color, line thickness, and fill color. 

These data values should not be specifically defined for all types of
layers. Polygon layers may have the three values mentioned above,
whereas an arc layer may only need the first two. The type of layer that
is being drawn will know what values are in the data_values list.

There would then have to be an appropriate dialog to allow these values
and ranges to be set.

Thoughts?

--jonathan