function drawDividers(gd, ax, opts) {
    var cls = ax._id + 'divider';
    var vals = opts.vals;

    var dividers = opts.layer.selectAll('path.' + cls)
        .data(vals, tickDataFn);

    dividers.exit().remove();

    dividers.enter().insert('path', ':first-child')
        .classed(cls, 1)
        .classed('crisp', 1)
        .call(Color.stroke, ax.dividercolor)
        .style('stroke-width', Drawing.crispRound(gd, ax.dividerwidth, 1) + 'px');

    dividers
        .attr('transform', opts.transFn)
        .attr('d', opts.path);
}
Draw axis dividers

@param {DOM element} gd
@param {object} ax (full) axis object
 - {string} _id
 - {string} showdividers
 - {number} dividerwidth
 - {string} dividercolor
@param {object} opts
- {array of object} vals (calcTicks output-like)
- {d3 selection} layer
- {fn} path
- {fn} transFn