Hi
I wrote little code to do this job. I post it here.
Pooya
Server Side Code:
    private void GenerateChart()
    {
        string[] source = Request.Form["s"].Split(',');
        Image[] ImageSource= new Image[source.Length];
        for (int ipos = 0; ipos < source.Length; ipos++)
        {
            ImageSource[ipos] = Base64ToImage(source[ipos]);
        }
        int OutputHeight=0;
        int OutputWidth=ImageSource[0].Width;
        for (int ipos = 0; ipos < source.Length; ipos++)
        {
            OutputHeight += ImageSource[ipos].Height;
        }
        var target = new Bitmap(OutputWidth, OutputHeight, PixelFormat.Format32bppArgb);
        var graphics = Graphics.FromImage(target);
        int Top = 0;
        for (int ipos = 0; ipos < source.Length; ipos++)
        {
            graphics.DrawImage(ImageSource[ipos], 0, Top);
            Top += ImageSource[ipos].Height;
        }
        Response.ClearHeaders();
        string base64;
        MemoryStream memory =new MemoryStream();
        target.Save(memory, ImageFormat.Png);
        base64 = Convert.ToBase64String(memory.ToArray());
        memory.Close();
        memory = null;
        
        Response.Write(base64);
    }
    private Image Base64ToImage(string base64String)
    {
        // Convert Base64 String to byte[]
        byte[] imageBytes = Convert.FromBase64String(base64String);
        MemoryStream ms = new MemoryStream(imageBytes, 0,
          imageBytes.Length);
        // Convert byte[] to Image
        ms.Write(imageBytes, 0, imageBytes.Length);
        Image image = Image.FromStream(ms, true);
        return image;
    }
Client Side Code:
function ExportToImage(ParentDiv) {
    var base64="";
    var charts=$(ParentDiv + " canvas");
    var no=charts.length;
    var ipos;
    for (ipos = 0; ipos < no; ipos++) {
        alert(charts[ipos].toDataURL("image/png").split(',')[0]);
        base64 += charts[ipos].toDataURL("image/png").split(',')[1];
        if (ipos < no - 1) base64 += ",";
    }
    $.ajax({
        url: "tsev2/chart/img/merge.aspx",
        type: "POST",
        cache: false,
        dataType: "text",
        data: { s: base64 },
        success: function (data) {
            var Result='data:image/png;base64,' + data;
            // Use Result in your code
        }
    });
}