Cut to only 5% free space on the top of the graphs

[benallard]

https://echarts.apache.org/en/option.html#yAxis.boundaryGap

Closes #1440

[codecov]

Codecov Report

Merging #1443 into master will decrease coverage by 0.38%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1443      +/-   ##
==========================================
- Coverage   81.05%   80.67%   -0.39%     
==========================================
  Files          26       27       +1     
  Lines        2344     2313      -31     
  Branches      362      362              
==========================================
- Hits         1900     1866      -34     
- Misses        351      354       +3     
  Partials       93       93              

Impacted Files	Coverage Δ
locust/runners.py	79.36% <0.00%> (-1.06%)	⬇️
locust/web.py	91.01% <0.00%> (-0.48%)	⬇️
locust/rpc/zmqrpc.py	86.20% <0.00%> (-0.46%)	⬇️
locust/stats.py	88.33% <0.00%> (-0.46%)	⬇️
locust/util/cache.py	90.90% <0.00%> (-0.40%)	⬇️
locust/user/task.py	96.66% <0.00%> (-0.06%)	⬇️
locust/rpc/protocol.py	100.00% <0.00%> (ø)
locust/__init__.py	100.00% <0.00%> (ø)

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[cyberw]

Cool! Thanks!