Returns a new copy of the BreakdownSet with another BreakdownSet subtracted from it. There must be a corresponding Breakdown for each Breakdown in both sets for this function to succeed. The metadata Breakdowns within the set are subtracted and the results combined into a new BreakdownSet. 

This feature is similar to the BreakdownSet divide function and can be used to create  meaningful Breakdown data of two metrics when the output value of the KPI formula is the value of one metric subtracted from another, then the output metadata could be on BreakdownSet subtracted from the other. 

var opened = metrics.get('Issues Opened');
var closed = metrics.get('Issues Closed');

var burndown = opened.latest(0) - closed.latest(0);

var openedMeta = opened.breakdown();
var closedMeta = closed.breakdown();

var breakdown = openMeta.subtract(closedMeta);

return {
   value : burndown,
   breakdown : breakdown

};
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