improve sigmaplot label handling

[baggepinnen]

No description provided.

[JuliaControlBot]

This is an automated message.
Plots were compared to references. 9/11 images have changed, see differences below.
After pulling this PR, please update the reference images by creating a PR to ControlExamplePlots.jl here.

Difference	Reference Image	New Image
✔️ 0.003
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✔️ 0.001
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✔️ 0.0
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❌ 0.042
✔️ 0.006
✔️ 0.011

[codecov]

Codecov Report

Merging #631 (ac15b75) into master (9f4c32c) will decrease coverage by 0.01%.
The diff coverage is 90.90%.

@@            Coverage Diff             @@
##           master     #631      +/-   ##
==========================================
- Coverage   86.69%   86.68%   -0.02%     
==========================================
  Files          31       31              
  Lines        3240     3245       +5     
==========================================
+ Hits         2809     2813       +4     
- Misses        431      432       +1     

Impacted Files	Coverage Δ
src/plotting.jl	85.12% <90.90%> (-0.06%)	⬇️

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[albheim]

LGTM