How do charts know how to update themselves?

Let’s say I have a chart with a specific dimension and group, such as:

var gender = xf.dimension(function (d) { return d.gender; }),
    genders = gender.group().reduceSum(function (d) { return d.interactions; })

Then I have another chart, which when interacted with (perhaps a pie chart click), it alters the first chart. How does the first chart know that it needs to update based on the value click on the pie chart? I’d imagine it must somehow know about the interactions property in order to rerun the reduce function such as:

// this runs inside the chart
group = group.reduceSum(function (d) {
  if (d.key == valueClicked) return d.interactions;
  else return 0;
})

Do interactions in DC.js actually involve rerunning the reduce functions or does it do it in some other way?

[gordonwoodhull]

In fact, one dimension’s filter will only change the groups on other dimensions, and not the groups on that dimension. It’s documented, but the documentation is very terse.

Note: a grouping intersects the crossfilter’s current filters, except for the associated dimension’s filter. Thus, group methods consider only records that satisfy every filter except this dimension’s filter. So, if the crossfilter of payments is filtered by type and total, then group by total only observes the filter by type.

https://github.com/crossfilter/crossfilter/wiki/API-Reference#dimension_group

I’m going to close this, because there isn’t really a dc.js request here. But hopefully this will help others.

[lmatteis]

I did not know that calling dimension.filter() for one dimension would also change other groups! Wish this was better documented on crossfilter’s site. Thanks this really cleared it for me.