This tool is what the standard MetaTrader 5 objects miss. To edit a formed line, click on it (at the edges or in the center of the line) until a dotted line appears. Then move the cursor without pressing a button.

The indicator has a simple animated button. Its size and transparency changes, when the mouse pointer is moved closer to it.

This implementation is mostly a training material and an attempt to inspire MetaTrader 5 developers to add the same functionality to the standard set of MetaTrader 5 objects. It is not implemented as a class. The following features have not been implemented: saving and passing of parameters in case of timeframe switch, multi-channel and smoothing functionality, channel extension to the right of the last bar. A version with full functionality can be implemented on demand.

This is a cross-platform code, which can also be used in MetaTrader 4. I apologize for not adding comments.

The arc is drawn according to the following formula:

where a, b, c are coefficients, and n is the bar number.

In fact, it is a polynomial of degree 2. Three points are enough to calculate the a, b, c coefficients.