Fixed index issue when getting scalar field value for a node.

[PProfizi]

Issue #219, The code used the index from enumerating the nodes instead of using the index_to_id function when retrieving the scalar value associated with a node in Plotter.add_label().

[akaszynski]

Let's add a unit test here.

Also, when you get a chance, it would be great to add code coverage so we can tell if lines or tested or not.

[PProfizi]

I could not see how to test this properly as this is really about displaying the right value in the plot.
As for test coverage it is among the things I want to implement in our pipelines as soon as I have time. I'll open an improvement issue.

[codecov]

Codecov Report

Merging #221 (fb867be) into master (01b4ec6) will increase coverage by 0.15%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #221      +/-   ##
==========================================
+ Coverage   85.45%   85.61%   +0.15%     
==========================================
  Files          52       52              
  Lines        5650     5650              
==========================================
+ Hits         4828     4837       +9     
+ Misses        822      813       -9