Shared by uniform_subplot_spacing_inputs_ui() and reset_plotly_inputs()
so the controls start and reset to the same values. A subplot.margin entry
sets both directions at once (linePlot uses it for its tighter default);
subplot.margin.x/subplot.margin.y override it per direction.
Examples
subplot_spacing_defaults(NULL)
#> $x
#> [1] 0.03
#>
#> $y
#> [1] 0.1
#>
subplot_spacing_defaults(list(subplot.margin = 0.05, subplot.margin.y = 0.2))
#> $x
#> [1] 0.05
#>
#> $y
#> [1] 0.2
#>