Anchor to Bars checkbox is located in the Format Drawing menu, on the Properties tab.
When enabled (by default), drawings are anchored to bars on a chart and will not change their position when new bars are added.
When disabled, drawings are anchored to date/time value and can be extended to right as new bars are added until the end date/time appears on a chart.
Note: Set the end point to a future date to have the drawing auto-extended automatically.