plot improvements and fixes

[Tantalor93]

new features and changes:

• svg plot format is now supported and is the new default
• improved readability of all line plot plots by overlaying scatter plot more clearly

fixes:

• line graphs are now properly relative to the start of the benchmark

breaking changes in public interfaces:

• Datapoint.Duration is now time.Duration
• Benchmark.PrintReport now also accepts the time when Benchmark started

[codecov]

Codecov Report

Attention: 12 lines in your changes are missing coverage. Please review.

Comparison is base (62e8d35) 79.38% compared to head (1a38651) 79.44%.

Files	Patch %	Lines
cmd/plot.go	87.30%	4 Missing and 4 partials ⚠️
cmd/report.go	57.14%	3 Missing ⚠️
cmd/root.go	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #204      +/-   ##
==========================================
+ Coverage   79.38%   79.44%   +0.05%     
==========================================
  Files           7        7              
  Lines        1140     1143       +3     
==========================================
+ Hits          905      908       +3     
  Misses        189      189              
  Partials       46       46              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.