User Guide
Synopsis:
These two videos explain why you have to take  the existing composition of  your portfolio into account whenever you decide to buy  a new stock or sell  an old one ; because your decsion could  significantly change  the risk and return profile , which may or may not in line with your investment goal, horizon and risk tolerance.
One simple way to see the impact of a new position on the whole portfolio is to run portfolio without the new position AND compared the results of the new (hypothetical) portfolio with the position to be included and make sure the return and risk profile are acceptable and aligned with with your investment objective .
Please see the comprehensive guide and  illustration in the User Guide in PDF below  the video