[R-package] fixed NOTE about unrecognized global variable

[jameslamb]

This PR fixes a new R CMD CHECK note introduced in #2803 . See #2803 (comment) for background.

The note is:

* checking R code for possible problems ... NOTE
multiple.tree.plot.interpretation: no visible binding for global
  variable ‘bar_color’
Undefined global functions or variables:
  bar_color

[StrikerRUS]

@jameslamb Thanks!

I can say nothing about the approach itself, but can confirm that the issue is eliminated:

* checking R code for possible problems ... OK

[jameslamb]

@jameslamb Thanks!

I can say nothing about the approach itself, but can confirm that the issue is eliminated:

* checking R code for possible problems ... OK

thanks! The approach is the same we've taken for other references in data.table.

I use this approach in every R package that has non-standard evaluation like those that use data.table or ggplot2. It's something that prominent R developers recommend.

It's funny, if you look in the documentation for ?globalVariables in R (which ships with R by default), it literally says:

For globalVariables, the names supplied are of functions or other objects that should be regarded as defined globally when the check tool is applied to this package

So it's basically like using // NOLINT to suppress false positives from a linter.

Given all this, I think it's ok to merge without another review, assuming CI succeeds.

[github-actions]

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