Fix histogram rotation to keep y as first dimension

[codecov[bot]]

Codecov Report

Merging #101 (5968833) into master (f347c8c) will decrease coverage by 0.14%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #101      +/-   ##
==========================================
- Coverage   76.28%   76.14%   -0.15%     
==========================================
  Files          26       25       -1     
  Lines        2703     2691      -12     
==========================================
- Hits         2062     2049      -13     
- Misses        641      642       +1     

Flag	Coverage Δ
unittests	76.14% <100.00%> (-0.15%)	:arrow_down:

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Impacted Files	Coverage Δ
traja/plotting.py	54.58% <100.00%> (+0.07%)	:arrow_up:
traja/dataset/dataset.py	98.42% <0.00%> (-1.58%)	:arrow_down:
traja/__init__.py

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[JustinShenk]

Closes #100