The polyline drawing tool is a way to draw a custom shape while still using straight lines. Any shape made from straight lines can be drawn and once the last point is connected with the first point of the figure, the shape will be completed.
In Style property dialog it is possible to change the appearance of a polyline:
Sets the polyline border color and thickness.
Toggles the visibility, color and opacity level for the polyline background.
In visibility properties dialog you can switch the Polyline tool displaying on charts of different timeframes: