Both the Sencha team and the Ext.NET team can no longer reproduce this issue as of Ext.NET 4.x.

Please let us know if you run into the same problem with Ext.NET 4.x (or higher) and we will reopen this Issue for investigation. Going forward, support for this issue can only be provided for Ext.NET 4.x (or higher).

The test case suitable for Ext.NET 4.4.0 (ExtJS 6.5.1) and C# would be:

<%@ Page Language="C#" %>

<script runat="server">
    public void Page_Load(object sender, EventArgs e)
    {
        if (!X.IsAjaxRequest)
        {
            store.DataSource = new List<object>()
            {
                new { XField = "Jan", Value = 0.75 },
                new { XField = "Feb", Value = 0.8 },
                new { XField = "Mar", Value = 0.85 },
                new { XField = "Apr", Value = 0.9 },
                new { XField = "May", Value = 0.8 },
                new { XField = "Jun", Value = 0.75 },
                new { XField = "Jul", Value = 0.85 },
            };

            store.DataBind();
        }
    }
</script>

<!DOCTYPE html>
<html>
<head id="Head1" runat="server">
    <title>Test case for thread 23521.</title>
</head>
<body>
    <script type="text/javascript">


</script>
    <form id="Form1" runat="server">
    <ext:ResourceManager ID="ResourceManager1" runat="server" ScriptMode="Debug" Theme="Gray"
        DisableViewState="true" />
    <ext:Panel runat="server" Title="Chart" Width="400" Height="400" Layout="FitLayout">
        <Items>
            <ext:CartesianChart runat="server">
                <Axes>
                    <ext:NumericAxis Position="Left" Fields="Value">
                        <Label>
                            <Renderer Handler="return Ext.util.Format.number(value * 100, '0') + '%' ;">
                            </Renderer>
                        </Label>
                    </ext:NumericAxis>
                    <ext:CategoryAxis Position="Bottom" Fields="XField">
                    </ext:CategoryAxis>
                </Axes>
                <Series>
                    <ext:LineSeries XField="XField" YField="Value">
                    </ext:LineSeries>
                </Series>
                <Store>
                    <ext:Store runat="server" ID="store">
                        <Model>
                            <ext:Model runat="server">
                                <Fields>
                                    <ext:ModelField Name="XField">
                                    </ext:ModelField>
                                    <ext:ModelField Name="Value">
                                    </ext:ModelField>
                                </Fields>
                            </ext:Model>
                        </Model>
                    </ext:Store>
                </Store>
            </ext:CartesianChart>
        </Items>
    </ext:Panel>
    </form>
</body>
</html>