function approxTitleDepth(ax) {
    var fontSize = ax.title.font.size;
    var extraLines = (ax.title.text.match(svgTextUtils.BR_TAG_ALL) || []).length;
    if(ax.title.hasOwnProperty('standoff')) {
        return fontSize * (CAP_SHIFT + (extraLines * LINE_SPACING));
    } else {
        return extraLines ?
            fontSize * (extraLines + 1) * LINE_SPACING :
            fontSize;
    }
}
Approximate axis title depth (w/o computing its bounding box)

@param {object} ax (full) axis object
 - {string} title.text
 - {number} title.font.size
 - {number} title.standoff
@return {number} (in px)