Use yscale --> _PlotScaleFunc in some plot recipes

[baggepinnen]

closes #798

[JuliaControlBot]

This is an automated message.
Plots were compared to references. 11/11 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
❌ 0.048
✔️ 0.011
✔️ 0.001
✔️ 0.001
✔️ 0.006
✔️ 0.001
✔️ 0.0
❌ 0.034
❌ 0.06
✔️ 0.0
⚠️ 0.017

[codecov]

Codecov Report

Merging #799 (3018ea6) into master (77e2eaa) will decrease coverage by 97.15%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #799       +/-   ##
==========================================
- Coverage   97.14%   0.00%   -97.15%     
==========================================
  Files           4       3        -1     
  Lines         315     313        -2     
==========================================
- Hits          306       0      -306     
- Misses          9     313      +304     

Impacted Files	Coverage Δ
src/root_locus.jl	0.00% <0.00%> (-97.78%)	⬇️
src/timeresp.jl	0.00% <0.00%> (-97.23%)	⬇️
src/simulators.jl	0.00% <0.00%> (-87.50%)	⬇️
src/ControlSystems.jl

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.