[feature][API] add SaveAsImage()

Runtime/Internal/BaseChart.API.cs

@@ -588,5 +588,15 @@ namespace XCharts.Runtime
             SerieHelper.GetItemColor(out color, out toColor, serie, null, m_Theme);
             return color;
         }
+
+        /// <summary>
+        /// 保存图表为图片。
+        /// </summary>
+        /// <param name="imageType">type of image: png, jpg, exr</param>
+        /// <param name="savePath">save path</param>
+        public void SaveAsImage(string imageType = "png", string savePath = "")
+        {
+            StartCoroutine(SaveAsImageSync(imageType, savePath));
+        }
     }
 }