Add peers to trend plot

[matt-dray]

Close #76.

• Added a line per peer onto the trend plot.
• Tweaked alpha (transparency) across rates plots for consistency (dark grey for peers, light grey for everyone else).
• Coloured the whole line red for the chosen unit in trend the chart (removed the red-higlighted baseline-year point).

Legibility is highly dependent on the selected TPMA, which will stretch/shrink the y-axis. At best we get some vertical separation:

But that's not always true when the y-axis is stretched given funnel properties:

Click to enlarge the images. Happy to demonstrate/talk this through