simple question on Charting, first 20 bars missing?

[redartajnin]

Just created a simple indicator to plot the closing price of each bar, on the chart, the plot always starts from the 21st bar, the first 20 bars are not plotted, why is that?

protected override void Initialize()
{
Add(new Plot(Color.FromKnownColor(KnownColor.Orange), PlotStyle.Line, "Plot0"));
Overlay = false;
}

/// <summary>
/// Called on each bar update event (incoming tick)
/// </summary>
protected override void OnBarUpdate()
{
// Use this method for calculating your indicator values. Assign a value to each
// plot below by replacing 'Close[0]' with your own formula.
Plot0.Set(Close[0]);
}