Thank for using Syncfusion products.
We are able to achieve the reported Tooltip requirement with the help of ChartTrackballBehavior. Please refer the below code example which shows the tooltip in series.
ChartTrackballBehaviorExt trackBall = new ChartTrackballBehaviorExt();
trackBall.LabelStyle = new ChartTrackballLabelStyle();
trackBall.LabelStyle.BackgroundColor = Color.Gray;
trackBall.LabelDisplayMode = TrackballLabelDisplayMode.FloatAllPoints;
trackBall.ShowLine = false;
We have prepared a sample based on your requirement. Please download the sample in the following location.
Please let us know if you have any further assistance on this.