[R-package] factor out {ggplot2}

[jameslamb]

Today the R package has a Suggests dependency on {ggplot2}. This is only for one demo, demo/leaf_stability.R. This pull request proposes factoring it out and replacing it with base R code.

This will allow us to cut out this R CMD check NOTE:

• checking package dependencies ... NOTE
  Package suggested but not available for checking: ‘ggplot2’

More importantly, it's an expensive dependency that anyone installing the Suggests dependencies of {lightgbm}has to pay for.

Some of these plots look different from the comments around them, but this PR does not address that...it can be addressed in #1944 . This PR is limited to just factoring out {ggplot2}, to make the package as lightweight as possible when it goes to CRAN (work-in-progress, #629 ).

Comparisons

I've included before and after screenshots for each plot below. The "Depth Density" plots look noticeably different, I think because of changes in how geom_density() works in {ggplot2}. demo/leaf_stability.R was written more than 3 years ago.

Thanks to @alistaire47 for helping me with {ggplot2} stuff 😀

plot 1

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plot 2

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plot 3

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plot 4

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plot 5

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plot 6

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plot 7

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plot 8

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[StrikerRUS]

LGTM for the idea to drop heavy dependency!

[StrikerRUS]

@jameslamb Can we now remove these lines?
https://github.com/microsoft/LightGBM/blob/50560573487e677037d392bfc0411e178ea80e6f/.ci/test_r_package.sh#L117-L118
https://github.com/microsoft/LightGBM/blob/50560573487e677037d392bfc0411e178ea80e6f/.ci/test_r_package_windows.ps1#L141

[jameslamb]

oooooooooooooooo yes we can! I forgot about those. Yes I'll remove them right now. We can always add back if we introduce new Suggests dependencies (like whenever #1944 is picked up, for rmarkdown) but for now it would be better to simplify those files.

I'll open a PR right now, thanks for the suggestion!

[github-actions]

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