Accepted answer

Do you need to redefine the extent after the initial call where the horizons are generated?

If not, try something like this:"body")
  .attr("class", "horizon")
  .attr("id", function(d){return d.toString();})
             .extent(function(d,i){  //use this function to change extents
                   if (d.toString() == 'Whatever the name is'){
                        return [0,1000];
                   } else {
                        return [ <default min>, <default max>]

You can either set your own default extent, or use a function like d3.min()/d3.max() to get the value for the horizons. You just need to return an array with two elements.

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