improve rlocusplot

[baggepinnen]

closes #611

[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
✔️ 0.003
✔️ 0.0
✔️ 0.001
✔️ 0.0
✔️ 0.0
❌ 0.042
✔️ 0.006
✔️ 0.012

[codecov]

Codecov Report

Merging #612 (dcc7f99) into master (094983a) will decrease coverage by 0.00%.
The diff coverage is 90.90%.

❗ Current head dcc7f99 differs from pull request most recent head d059caa. Consider uploading reports for the commit d059caa to get more accurate results

@@            Coverage Diff             @@
##           master     #612      +/-   ##
==========================================
- Coverage   86.81%   86.81%   -0.01%     
==========================================
  Files          31       31              
  Lines        3239     3245       +6     
==========================================
+ Hits         2812     2817       +5     
- Misses        427      428       +1     

Impacted Files	Coverage Δ
src/pid_design.jl	78.73% <90.90%> (+0.16%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 094983a...d059caa. Read the comment docs.

[baggepinnen]

Hmm, the new default limits are no good :/