Let y-axis of trend charts start at the minimum value and not at zero

[uhafner]

For most charts it makes sense to let the Y-Axis start at the chart minimum value and not at zero.

[codecov]

Codecov Report

Merging #138 (c668bbf) into master (de10ad8) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #138   +/-   ##
=========================================
  Coverage     64.28%   64.28%           
  Complexity        5        5           
=========================================
  Files             3        3           
  Lines            42       42           
  Branches          3        3           
=========================================
  Hits             27       27           
  Misses           15       15           

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