Plot counts per lineage by region with matplotlib instead of altair

[huddlej]

I originally plotted counts of HA sequences by lineage and region with altair, with the plan to make the figure interactive. However, we end up most often including these figures in static reports. We also would like to automate the creation of these figures in our automated Nextstrain builds, but altair is not available in the Nextstrain environment. Instead of adding altair to the environment, we should just make these plots with matplotlib in a script.