Yeah this will probably end up being one of our benchmarks -- it's commonly used in academic literature to measure abnormal returns.

The common-signal would just be the general stock market performance. S&P is one way to approximate "the market". The idea being that you want a plot to show you how well they're doing in terms of market performance, not in terms of absolute share price (which is kind of meaningless here). Otherwise every shock to the general market ends up getting rendered in every line in the plot which is pretty distracting and makes it hard to see what's going on.

I'm not entirely sure either but you could use some combination of dS/dt dI/dt where S is the senator and I is the market?