Region number should be exposed for stacked bar charts

[gwatts]

Exposing the region number would allow lookup values to be keyed against the region rather than just the value or offset

Eg. for a stacked bar chart where you want the first series to have prefix "first" and the second series "second":

        $('#mysparkline').sparkline('html', 
            {
                type: 'bar', 
                tooltipFormat: '{{region:prefixes}} {{value}}', 
                tooltipValueLookups: {prefixes: {0: 'first', 1: 'second'}}
                }
        );