Hi Stephan,
Thank you for your interest in Syncfusion products.
I am not able to reproduce the issue that you mentioned here.I have created a simple sample with your code snippet for adding custom button in the toolbar. Here it adds the button at the last position. But you can set the position of the new button in the toolbar by using the insert method by specifying location. Please refer the below code snippet:
[C#]
Syncfusion.Web.UI.WebControls.Tools.ToolBarItem tbi = new Syncfusion.Web.UI.WebControls.Tools.ToolBarItem();
tbi.ButtonType = Syncfusion.Web.UI.WebControls.Tools.ToolBarControl.ToolBarButtonType.Regular;
tbi.Text = "Export values to Excel";
tbi.ImagePath = "Image/smile.bmp";
tbi.ToolTip = "Export values to Excel";
tbi.AppearanceMode = Syncfusion.Web.UI.WebControls.Tools.ToolBarControl.AppearanceMode.ImageOnly;
// this.ChartWebControl1.ToolBar.Items.Add(tbi);
//Set Position for the button:
this.ChartWebControl1.ToolBar.Items.Insert(4,tbi);
Here is the sample for your reference:
http://websamples.syncfusion.com/samples/Chart.Web/6.2.0.40/73411_ToolBar/main.htmPlease try this and let me know if this helps.
Regards,
G.Yavana