if you want to dynamically change the height of iframe base on panel within 
Grafana using JavaScript, you can achieve this by this:

 <iframe id="embeddedFrame" src="http://your.external.web.page.url" width="100%"> </iframe>
<script>
var iframe = document.getElementById("embeddedFrame");

    function setIframeHeight() {
        iframe.style.height = iframe.parentElement.offsetHeight-5 + "px";
    }

    // Call setIframeHeight when the iframe content is loaded
    iframe.onload = setIframeHeight;

    // Also, call setIframeHeight when the window is resized
    window.addEventListener("resize", setIframeHeight);
</script>