You need to initialize animation if the element is in viewport. Check isElementInViewport function on window scroll.
Sample js
function isElementInViewport (el) {
    //special bonus for those using jQuery
    if (typeof jQuery === "function" && el instanceof jQuery) {
        el = el[0];
    }
    var rect = el.getBoundingClientRect();
    return (
        rect.top >= 0 &&
        rect.left >= 0 &&
        rect.bottom <= (window.innerHeight || document.documentElement.clientHeight) && /*or $(window).height() */
        rect.right <= (window.innerWidth || document.documentElement.clientWidth) /*or $(window).width() */
    );
}
var isAnimated = false;
window.onscroll = function(){
    if(isElementInViewport($('#myStat')) && !isAnimated){
        $('#myStat').circliful();
        isAnimated = true;
    }
}
How to tell if a DOM element is visible in the current viewport?