Wrong Zero Level in Genome Coverage

[donkirkby]

While working on fixing the contig alignment in genome coverage plots for #480, I noticed that the actual coverage levels are displayed incorrectly. The zero coverage level is in the centre of the contig's grey bar, not at the top. Here's an example of the current coverage plot where there the four quarters of the contig have coverage of 5, 10, 0, and 1. Obviously, the second quarter is more than twice the depth of the first quarter, and the fourth quarter isn't visible at all.

Here's what it should look like: