Please allow me referring you to What are the plans for extensibility in PI Coresight? because we consider this a must know before deciding for customizing PI Coresight using the extensibility. Please don't misunderstand this as an attempt to convince from not using extensibility. We still see value in using it but you should be aware that what you create today will very likely break with a certain update.
Please see PI Developers Club Webinar Series: Extending PI Visualization - how PI Coresight 2016 empowers you! to learn what it takes to create a custom symbol for PI Coresight.
was wondering, did you succeed to implement a stacked aera chart in coresight/PI Vision ?
We don't have an 'official' implementation of the stacked area chart on our github repository for custom symbols. I implemented the stacked area chart from AmCharts a year or so ago with a few tweaks to the code (my end goal was showing individual contributions to a roll-up attribute). Feel free to reach out over in the PI Visualization Development section of PI Square if you have any questions on implementing this.