Multiple models on one chart

### Expected Behavior

Is there an elegant and simple way to visualize a few trading models plotted on 1 chart? In fact a lot of projects like mine are centered around comparing various models (and even groups of models), so that would be handy to have a sugary way around this process, so:
1) individual models with notations on one chart
2) a group of models trading on the same account each having a proportional share of the capital

### Actual Behavior

works with just 1 model

The issue names no files, tests, or entry points. Start by reading the charting and backtesting APIs, then trace how a single model and account are currently handled. Done should support multiple annotated models on one chart and proportional capital sharing for models using the same account.

Written by the indexing model from the issue text.