Plot P/L triangles in absolute currency units

I have set the reference price for the cash PnL at the spread between two securities (I am backtesting a mean reversion strategy). However, in the html output of the plot I am getting the PnL in terms of the differnce in the spread as a function of the entry price of the spread when the trade was initiated. Instead, I want to get the PnL based off of the prices of the sum of the two securities (VIX futures) which make up the spread. This would look like `cash difference in spread / sum(security_1, security_2)`. I have tried to add an additional class which could be inherited from `Order` but it's not working out.

Any ideas on how to provide custom data points (column names of our data) to compute the PnL during `run()`?

Start by tracing how run() uses Order data to calculate PnL and how the HTML plot renders the result. Review the attempted custom Order approach and determine how column names or other custom data points could be supplied; done means the plot supports PnL in absolute currency units based on the sum of the two securities.

Written by the indexing model from the issue text.