@Grant: Thanks for the input. I agree that plotting is a critical feature. I thought about this issue a little more but still don't feel like I have the perfect solution. Essentially, there are two ways:
As fawce described, allowing you to add time-series plots in the existing backend
-covers quite some use cases
-history directly accessible
-not very general, many ways of plotting that do not find into standard time-series line plots.
As you described, giving you access to matplotlib.
-extremely general and feature rich right off the bat
-can't easily look at previous values (unless the user handles that case) as by default they will get overwritten.
-not easy to integrate into our current backtesting UI.
One idea would be to add functionality similar to batch_transform, that gets called periodically (say, weekly, but user-defined). Instead of doing some transform one is creating a matplotlib plot. This figure we save to a file (with a timestamp) and display in some way in the backend (maybe new tab as you suggested, can also be saved). Critically, you could be able to look at plots at a previous point in time.
I'm not sure, am I overemphasizing the point that previous plots would get lost? To my mind, that's really important.
Interested to hear your thoughts!