As of today the library has no built in method to calculate average entry price. It is usually used in startegies like DCA and martiangle. use the followin code to calculate it

entry_price = self._broker.last_price / (1 + self.position.pl_pct)
# OR
entry_price = self._broker.last_price - (self.position.pl / self.position.size)

Then you can plot it with take profit to see a good overview how your startegy is going
image
Take care that that values are updated after next() cycle as simulating broker execusion. You can keep track manually but the above method is aligned with our library logic and accuracy
I am open for discussion