Improve plot_blooddata to include BPR when it's too high

[mathesong]

When the Blood-to-Plasma ratio is too high, the BPR curve can disappear from the plots. A fix is needed to show it better.

One of the following two options:

• Show the Plasma-to-Blood ratio instead (if >50% of the curve is obscured)
• Include a little panel at the top (if >80% of the curve is obscured)

I'm leaning towards the former...

[mathesong]

Ok, done. Couldn't resist.