allow passing None for missing vals in timeseries plot

[knaaptime]

• allow None for missing args
• allow alpha parameter

[codecov[bot]]

Codecov Report

Merging #297 (f7cfb50) into master (7a24885) will increase coverage by 0.2%. The diff coverage is 100.0%.

@@           Coverage Diff            @@
##           master    #297     +/-   ##
========================================
+ Coverage    68.8%   69.0%   +0.2%     
========================================
  Files          14      14             
  Lines        1189    1192      +3     
========================================
+ Hits          818     823      +5     
+ Misses        371     369      -2     

Impacted Files	Coverage Δ
geosnap/_community.py	72.8% <100.0%> (+0.2%)	:arrow_up:
geosnap/analyze/analytics.py	85.9% <0.0%> (+1.0%)	:arrow_up: