Small tweaks to the plot plugin

[chapulina]

Just a couple of tweaks that I noticed while reviewing gazebosim/gz-sim#270:

• The check in Chart.qml is needed to avoid warnings after a field is deleted from a plot
• Changed the title of the transport plotting plugin so it's different from the one in ign-gazebo.

@AmrElsersy , mind taking a look?

[codecov]

Codecov Report

Merging #125 into master will increase coverage by 0.15%.
The diff coverage is 47.82%.

@@            Coverage Diff             @@
##           master     #125      +/-   ##
==========================================
+ Coverage   63.07%   63.23%   +0.15%     
==========================================
  Files          20       20              
  Lines        2592     2592              
==========================================
+ Hits         1635     1639       +4     
+ Misses        957      953       -4     

Impacted Files	Coverage Δ
src/PlottingInterface.cc	56.52% <0.00%> (+0.80%)	⬆️
src/plugins/world_stats/WorldStats.cc	97.27% <100.00%> (+0.10%)	⬆️

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[claireyywang]

LGTM