dc.js pie-chart combine 2 columns

问题

I have a table.

year,continent,state,export_value,import_value,all
2013,Африка,Angola,94061754,220518.54,94282272.54
2013,Африка,Burundi,4136818,1239175.91,5375993.91
2013,Африка,Benin,3251643.49,65421.6,3317065.09

I need to make a pie chart using dc.js:

Here is what I tried:

    impexDim            = ndx.dimension(function (d) {return ((d.export_value > 0 ? 'Export' : 'Import') 
                                        && (d.import_value > 0 ? 'Import' : 'Export')) ;}),

    groupeimpexDim      = impexDim.group().reduceSum(function(d) {return +d.all}),

How do I display the columns export_value and import_value as slices of the pie chart?

来源：https://stackoverflow.com/questions/42135743/dc-js-pie-chart-combine-2-columns