Add an adjustable zIndex to legend:
(add on line 8087): zIndex = options.zIndex || 10;
(change on line 8403): .attr({ zIndex: zIndex })

Add itemMarginLeft and itemMarginRight:
(change on line 8377):
if (horizontal) {
itemX += itemMarginLeft + itemWidth + itemMarginRight;
} else {
itemY += itemMarginTop + itemHeight + itemMarginBottom;
}