When Series are created programmatically, Axes settings are not preserved after postback. The workaround is to create an empty series in the markup. For example:

        <telerik:RadHtmlChart ID="RadHtmlChart1" runat="server">
...
            <PlotArea>
                <YAxis>...</YAxis>
                <XAxis>...</XAxis>
                <Series>
                    <telerik:ScatterLineSeries Name=" ">
                        <Appearance FillStyle-BackgroundColor="Transparent"></Appearance>
                    </telerik:ScatterLineSeries>
                </Series>
            </PlotArea>
        </telerik:RadHtmlChart>