The current behavior of the RadHtmlChart preserves only the bound datasource, however it doesn't store programmatically added series items after a postback.

The workaround is instead of creating and adding series items programmatically, create a datatable and pass it as a datasource to the chart. For example:
ASPX:
<telerik:RadHtmlChart runat="server" ID="RadHtmlChart1" Width="600px" Height="400px">
</telerik:RadHtmlChart>
C#:
protected void Page_Load(object sender, EventArgs e)
{
    DataTable dt = new DataTable();
    dt.Columns.Add("id");
    dt.Columns.Add("xValues");
    dt.Columns.Add("yValues");
    for (int i = 0; i < 5; i++)
    {
        dt.Rows.Add(i, (10 + i).ToString() + " Apr", 20 + i);
    } 
    ScatterLineSeries sls1 = new ScatterLineSeries();
    sls1.LabelsAppearance.DataFormatString = "{0:d}";
    sls1.DataFieldX = "xValues";
    sls1.DataFieldY = "yValues";
  
    RadHtmlChart1.PlotArea.Series.Add(sls1);
    RadHtmlChart1.DataSource = dt;
    RadHtmlChart1.DataBind();
}