As a workaround you can either use Lightweight render mode or Server group load mode.

RadHtmlChart does not cache the correct size when placed inside initially hidden iframe.

Workaround:

            function redrawChart() {
                kendo.drawing.util.TextMetrics.current._cache = new kendo.drawing.util.LRUCache(1000);
                $find("<%=RadHtmlChart1.ClientID%>").get_kendoWidget().redraw();
            }