New time series created by 2 existing instruments

Hi,

I think it would be really useful if one could create a graph that is based on 2 already existing instruments.

For example: I like to look at a chart where I have the performance of my portfolio and a benchmark index. It would be nice to be able to add a third graph to the chart that would be the difference between my portfolio and the benchmark.

Is this something that could be relatively easily added in a future update? I think it would be a useful tool to monitor your long term performance.