Plot estimates with zero counts differently

[dp-rice]

For estimates with no viral reads, don't plot the boxes, instead indicate that we have upper bounds on our estimates by plotting:

• Left-pointing triangles at the ~94th percentile
• A faint dashed line to the left edge of the plot

Note that for the plot by location, we will still plot the boxes since these estimates use information borrowed from the other locations.

One other minor change: we now give the vertical bar for the median the same alpha value as the other elements.

[dp-rice]

Final version: