Add net exposure line to the Long/Short Exposure plot

[justinlent]

Just subtract Short from Long and plot the difference as a timeseries.
Also a couple of minor formatting changes.

Long exposure = Green
Short exposure = Red
Difference line = Black dashed

Make Long exposure line positive number, and Short exposure line all negative numbers:
(input from @Jstauth) rather than showing both as positive (abs) we should show the long as positive and short as negative, then we could overlay the net exposure as a line)

[justinlent]

See conversation thread here for description of the bug and possible fixes: #301

[justinlent]

@adeychman just FYI for you