A line y=ax+b can be drawn through a set of points, and the distance from each point to the line will be approximately balanced.

To plot the HighLine and LowLine lines it uses the set of High and Low points, respectively, for the MyPeriod period.

If the CalculateDeflection parameter is enabled, then two more lines - HighDeflection and LowDeflection are placed on the chart. They display the possible price variation (mean square error). The hidden line b shows the value of the free variable in the line equation.

The MLS-HL4-Angle indicator calculates the value of a in points and indicates the inclination angle of the obtained line to the horizon.
The indicator displays the value of the initial price b and the rate of change of angle p relative to the tangent. If p>0, then the curve a turns left, so the angle tends to positive values. However, if p<0, then the curve a tries to turn right and tends the angle to negative values

The MLS.zip archive contains the math of calculation using the least squares method.