fix plot_trend bug

[iKintosh]

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Proposed Changes

Related Issue

Closes #612

Closing issues

[codecov-commenter]

Codecov Report

Merging #617 (201a2ac) into master (067f6ad) will increase coverage by 0.43%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #617      +/-   ##
==========================================
+ Coverage   84.12%   84.55%   +0.43%     
==========================================
  Files         118      118              
  Lines        5964     5964              
==========================================
+ Hits         5017     5043      +26     
+ Misses        947      921      -26     

Impacted Files	Coverage Δ
etna/analysis/plotters.py	22.14% <100.00%> (+6.06%)	⬆️

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[Mr-Geekman]

Can't we unite this test with previous? They are inherited from the same base class.

[iKintosh]

test_plot_trend

test_plot_bin_seg

test_plot_stl