textOnPath

// Based on http://stackoverflow.com/a/19122486/478725
// Text is rotated to be read begin -> end
// Variable width fonts will still be a bit off center

var angle = function(v1, v2) {
   ang = Math.atan2(v2.y - v1.y, v2.x - v1.x);
   return {
      radians: ang,
      degrees: ang * 180 / Math.PI
   };
};

function textOnPath( paper, message, path, fontSize, letterSpacing, kerning, geckoKerning) {
    // if fontSize or letterSpacing are undefined, they are calculated to fill the path
    // 10% of fontSize is usually good for manual letterspacing

    // Gecko, i.e. Firefox etc, inflates and alters the letter spacing
    var gecko = /rv:([^\)]+)\) Gecko\/\d{8}/.test(navigator.userAgent||'') ? true : false;

    var letters = [], places = [], messageLength = 0;
    for (var c=0; c < message.length; c++) {
        var letter = paper.text(0, 0, message[c]).attr({"text-anchor" : "middle"});
        var character = letter.attr('text'), kern = 0;
        letters.push(letter);

        if (kerning) {
            if(gecko && geckoKerning) {
                kerning = geckoKerning;
            }
            var predecessor = letters[c-1] ? letters[c-1].attr('text') : '';
            if (kerning[c]) {
                kern = kerning[c];
            } else if (kerning[character]) {
                if( typeof kerning[character] === 'object' ) {
                    kern = kerning[character][predecessor] || kerning[character]['default'] || 0;
                } else {
                    kern = kerning[character];
                }
            }
            if(kerning['default'] ) {
                kern = kern + (kerning['default'][predecessor] || 0);
            }            
        }

        messageLength = messageLength ? messageLength : letter.getBBox().width;
        messageLength += kern;
        places.push(messageLength);
        //spaces get a width of 0, so set min at 4px
        messageLength += Math.max(4.5, letter.getBBox().width);
    }

    if( letterSpacing ){
        if (gecko) {
            letterSpacing = letterSpacing * 0.83;
        }
    } else {
        letterSpacing = letterSpacing || path.getTotalLength() /  messageLength;
    }
    fontSize = fontSize || 10 * letterSpacing;
      
    for (c = 0; c < letters.length; c++) {
        letters[c].attr("font-size", fontSize + "px");
        p = path.getPointAtLength(places[c] * letterSpacing);
        //var rotate = 'R' + (p.alpha <= 180 ? p.alpha + 180 : p.alpha > 360 ? p.alpha - 360 : p.alpha )+','+p.x+','+p.y;
        var pp1 = path.getPointAtLength(places[c]*letterSpacing-0.1),
            pp2 = path.getPointAtLength(places[c]*letterSpacing+0.1),
            rotate = 'R' + angle(pp1, pp2).degrees +','+p.x+','+p.y;
        letters[c].attr({ x: p.x, y: p.y, transform: rotate });
    }
}