     var chunks = [];
        for (var j=0; j<ch.length; j++) {
            var lo = ch[j][1] / Nchunk;
            var hi = ch[j][2] / Nchunk;
            var wh = ch[j][3];
            //console.log('Chunk ' + lo + '+' + (hi-lo) + ' from ' + wh);
            chunk_lo.push(lo);
            chunk_hi.push(hi);
            chunks.push([lo, hi, wh]);
        }
        var lolo = d3.min(chunk_lo);
        var hihi = d3.max(chunk_hi);
        //console.log('overall range: ' + lolo + ' to ' + hihi);

        this