added timestamp import to matplotlib linecharts (#396)

[dorisjlee]

closes #396

also temporary fix for #398

[codecov]

Codecov Report

Merging #397 (bbdb803) into master (17dd0ee) will decrease coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #397      +/-   ##
==========================================
- Coverage   85.09%   85.05%   -0.05%     
==========================================
  Files          52       52              
  Lines        4033     4034       +1     
==========================================
- Hits         3432     3431       -1     
- Misses        601      603       +2     

Impacted Files	Coverage Δ
lux/vislib/matplotlib/LineChart.py	88.88% <100.00%> (+0.15%)	⬆️
lux/vislib/altair/ScatterChart.py	93.93% <0.00%> (-3.04%)	⬇️
lux/vislib/matplotlib/ScatterChart.py	75.58% <0.00%> (-1.17%)	⬇️

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