Fix graph generation in doc examples

[FredericWantiez]

Some of the graphs in the documentation are mixed up or do not show up properly, I think this should fix it.

[coveralls]

Pull Request Test Coverage Report for Build 2565088946

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 3 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.6%) to 64.717%

---

Files with Coverage Reduction	New Missed Lines	%
src/model.jl	3	88.1%

Totals
Change from base Build 2511252505:	-0.6%
Covered Lines:	332
Relevant Lines:	513

---

💛 - Coveralls

[codecov]

Codecov Report

Merging #65 (0c1eac5) into master (0bb8fde) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #65   +/-   ##
=======================================
  Coverage   65.30%   65.30%           
=======================================
  Files           7        7           
  Lines         513      513           
=======================================
  Hits          335      335           
  Misses        178      178           

---

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