transform svg element around its center

ts.js

// transform around center:
// formula: matrix(sx, 0, 0, sy, cx-sx*cx, cy-sy*cy)
var b = el.getBBox();
var center = [b.x + b.width/2, b.y + b.height/2];
var factor = 1.8;
el.attr('transform','matrix(' + factor + ', 0, 0, ' + factor + ', ' +
        (center[0] - factor * center[0]) +
        ',' + (center[1] - factor * center[1])+')');