Remove y-axis line from density, histogram, and dot plots

[utkarshpawade]

This PR removes the y-axis line from plots where the y-axis has no meaningful scale, addressing the inconsistency noted in #418.

Changes

• Added theme(axis.line.y = element_blank()) to 13 functions across 3 files
• Functions affected: mcmc_hist, mcmc_hist_by_chain, mcmc_dens, mcmc_dens_overlay, mcmc_violin, ppc_dens_overlay, ppc_dens, ppc_hist, ppc_freqpoly, ppc_dots, ppd_dens_overlay, ppd_dens, ppd_hist, ppd_dots, ppd_freqpoly
• Follows the same pattern already used in mcmc_dots()
• Updated all visual regression test snapshots (51 files)
• Added entry to NEWS.md

Testing

• All 1387 tests pass
• Visual regression tests updated and verified - y-axis line successfully removed
• No functional changes to plot data or calculations

Fixes #418

[Copilot]

Pull request overview

This PR updates bayesplot distribution-plot functions to remove the y-axis axis-line for plots where the y-axis is intentionally not meaningful (labels/ticks/title are already suppressed), and updates visual regression snapshots accordingly.

Changes:

• Added theme(axis.line.y = element_blank()) to multiple MCMC / PPC / PPD distribution plotting functions.
• Updated visual regression SVG snapshots to reflect the removed y-axis line.
• Added a NEWS entry describing the visual change.

Reviewed changes

Copilot reviewed 4 out of 51 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
R/ppd-distributions.R	Removes y-axis axis-line for selected PPD distribution plots (overlay, dots, freqpoly).
R/ppc-distributions.R	Removes y-axis axis-line for key PPC distribution plots (dens overlay, dens, hist, freqpoly, dots).
R/mcmc-distributions.R	Removes y-axis axis-line via internal helpers affecting MCMC hist/dens/violin/dots families.
NEWS.md	Notes the visual change in the development NEWS.
tests/testthat/_snaps/ppc-distributions/ppd-freqpoly-grouped-default.svg	Snapshot updated to reflect removed y-axis line.
tests/testthat/_snaps/ppc-distributions/ppd-freqpoly-default.svg	Snapshot updated to reflect removed y-axis line.
tests/testthat/_snaps/ppc-distributions/ppd-freqpoly-alpha-binwidth-size.svg	Snapshot updated to reflect removed y-axis line.
tests/testthat/_snaps/ppc-distributions/ppd-dots-quantile.svg	Snapshot updated to reflect removed y-axis line.
tests/testthat/_snaps/ppc-distributions/ppd-dots-quantile-binwidth.svg	Snapshot updated to reflect removed y-axis line.
tests/testthat/_snaps/ppc-distributions/ppd-dots-default.svg	Snapshot updated to reflect removed y-axis line.
tests/testthat/_snaps/ppc-distributions/ppd-dots-binwidth.svg	Snapshot updated to reflect removed y-axis line.
tests/testthat/_snaps/ppc-distributions/ppd-dens-overlay-default.svg	Snapshot updated to reflect removed y-axis line.
tests/testthat/_snaps/ppc-distributions/ppd-dens-overlay-bounds.svg	Snapshot updated to reflect removed y-axis line.
tests/testthat/_snaps/ppc-distributions/ppd-dens-overlay-alpha-size.svg	Snapshot updated to reflect removed y-axis line.
tests/testthat/_snaps/ppc-distributions/ppc-hist-default.svg	Snapshot updated to reflect removed y-axis line.
tests/testthat/_snaps/ppc-distributions/ppc-hist-binwidth.svg	Snapshot updated to reflect removed y-axis line.
tests/testthat/_snaps/ppc-distributions/ppc-freqpoly-grouped-default.svg	Snapshot updated to reflect removed y-axis line.
tests/testthat/_snaps/ppc-distributions/ppc-freqpoly-default.svg	Snapshot updated to reflect removed y-axis line.
tests/testthat/_snaps/ppc-distributions/ppc-freqpoly-alpha-binwidth-size.svg	Snapshot updated to reflect removed y-axis line.
tests/testthat/_snaps/ppc-distributions/ppc-dots-quantile.svg	Snapshot updated to reflect removed y-axis line.
tests/testthat/_snaps/ppc-distributions/ppc-dots-quantile-binwidth.svg	Snapshot updated to reflect removed y-axis line.
tests/testthat/_snaps/ppc-distributions/ppc-dots-default.svg	Snapshot updated to reflect removed y-axis line.
tests/testthat/_snaps/ppc-distributions/ppc-dots-binwidth.svg	Snapshot updated to reflect removed y-axis line.
tests/testthat/_snaps/ppc-distributions/ppc-dens-overlay-grouped-default.svg	Snapshot updated to reflect removed y-axis line.
tests/testthat/_snaps/ppc-distributions/ppc-dens-overlay-grouped-alpha-size.svg	Snapshot updated to reflect removed y-axis line.
tests/testthat/_snaps/ppc-distributions/ppc-dens-overlay-default.svg	Snapshot updated to reflect removed y-axis line.
tests/testthat/_snaps/ppc-distributions/ppc-dens-overlay-bounds.svg	Snapshot updated to reflect removed y-axis line.
tests/testthat/_snaps/ppc-distributions/ppc-dens-overlay-alpha-size.svg	Snapshot updated to reflect removed y-axis line.
tests/testthat/_snaps/ppc-distributions/ppc-dens-default.svg	Snapshot updated to reflect removed y-axis line.
tests/testthat/_snaps/mcmc-distributions/mcmc-violin-default.svg	Snapshot updated to reflect removed y-axis line.
tests/testthat/_snaps/mcmc-distributions/mcmc-hist-freq.svg	Snapshot updated to reflect removed y-axis line.
tests/testthat/_snaps/mcmc-distributions/mcmc-hist-default.svg	Snapshot updated to reflect removed y-axis line.
tests/testthat/_snaps/mcmc-distributions/mcmc-hist-by-chain-default.svg	Snapshot updated to reflect removed y-axis line.
tests/testthat/_snaps/mcmc-distributions/mcmc-hist-alpha.svg	Snapshot updated to reflect removed y-axis line.
tests/testthat/_snaps/mcmc-distributions/mcmc-dens-overlay-default.svg	Snapshot updated to reflect removed y-axis line.
tests/testthat/_snaps/mcmc-distributions/mcmc-dens-overlay-bounds.svg	Snapshot updated to reflect removed y-axis line.
tests/testthat/_snaps/mcmc-distributions/mcmc-dens-default.svg	Snapshot updated to reflect removed y-axis line.
tests/testthat/_snaps/mcmc-distributions/mcmc-dens-bounds.svg	Snapshot updated to reflect removed y-axis line.
tests/testthat/_snaps/mcmc-distributions/mcmc-dens-alpha.svg	Snapshot updated to reflect removed y-axis line.

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