Allow precisely setting FC/ATA limits

[ptgolden]

The only way to currently set them is by dragging along the plot

[ptgolden]

There would be two approaches to this:

1. Explicitly setting the bounds of the brush

2. Explicitly setting the size of the plot. This might mean treating other transcripts like they "don't exist", i.e. graying them out in the table like when they don't meet the p-value threshold. (This might be a good thing to do already when zooming).