    V.transformRect(r, matrix)

Transform a rectangle specified by `r` (an object with `x`, `y`, `width` and `height` properties) by an SVG transformation represented by `matrix`. This function is used internally in the V.prototype.bbox method to return a bounding box of an SVG element relative to another SVG parent element. However, this can be used elsewhere too, that's why it is publicly exposed. Whenever you need to transform a bounding box to a coordinate system of another element, use this function. To get the transformation `matrix` of the target element relative to which you want to tranform your rectangle, use e.g. the native SVG method: `var matrix = svgSourceElement.getTransformToElement(svgTargetElement)`.