[FIX] reimplement transformation for stacked bars
[FIX] reimplement transformation for stacked bars

In DHTMLX, stacked bars require a very different topology than
clustered, essentially it requires taking clustered data and "rotating
it".

This implement the transformation of the series descriptors
('group_list' sequence which sets up the key, label and color for each
series) and of the series itself (sets up a key for each possible
value of the aggregation field, with each key prefixed by the name of
the ordinate column).

To handle grouped *and clustered* graphs, which dhtmlx does not
natively support, might be able to reimplement this Excel/Numbers
hack:
http://peltiertech.com/WordPress/clustered-stacked-column-bar-charts/
by iterating over all this.columns (rather than just the ordinate)
and... not sure actually, creating a record for each of these columns
for each abscissa value, with all other fields set to 0? Maybe?


havt/openerp-web,havt/openerp-web,havt/openerp-web,akretion/openerp-web,havt/openerp-web,akretion/openerp-web,akretion/openerp-web,havt/openerp-web