@media screen and (max-width: 768px) {
  /*chartCSS*/
  .chart-container {
    position: relative; 
    width:90%; 
    height:70vh;
  }
}
/* 画面サイズの横幅が 769px 以上で適用 */
@media screen and (min-width: 769px) { 
  /*chartCSS*/
   .chart-container {
    position: relative; 
    width:90%; 
    height:70vh;
  }
}