private void bt_export4calcu_Click(object sender, EventArgs e)
{
PValue beforepoint = new PValue(double.Parse(tb_startvalue.Text), startDatetime, 0);
PValue afterpoint = new PValue(double.Parse(tb_endvalue.Text), endDatetime, 0);
PValue insertpoint = PValue.Interpolate(beforepoint, afterpoint, insertDatetime);
tb_insertvalue.Text = insertpoint.Value.ToString();
}