SamStats: GC coverage plot

[PedalheadPHX]

The GC coverage plot for ANBL6 test example is missing the coverage track for some reason

[PedalheadPHX]

The similar picard plot does have it so likely a version issue, when I tested with my most recent plot_samStats push this worked fine on the same sample

[PedalheadPHX]

Versus current output

[bryce-turner]

I'm noticing this for MM1S with RNA data that has a low median coverage of 2 +/- 1, pulled your most recent commits and the only diff is the updates for mapq support. I'll queue up some additional runs (which we need to do anyway) to see if there is a pattern here.