Add `grid()` args to tpar?

[grantmcdermott]

At present, we support a simple logic argument for automatically drawing a grid all plots via tpar(grid = TRUE).

But this doesn't give users much control over how their grids look, since it enforces the default colour ("lightgray"), line type ("dotted"), and width (par("lwd")).

It's not a huge deal, because users can adjust these settings for individual plots by passing grid = grid(...) in the tinyplot call.

tinyplot(
    sin(0:100/10), axes = "l",
    grid = grid(col = "hotpink", lty = "solid")
)

But... thinking ahead to our goal of enabling themes (#234), maybe it's a low cost intervention to define these grid elements globally in tpar(grid.col = 'lightgray", grid.lty = "dotted", grid. lwd = 1).

[zeileis]

I think it would be good to have control over this using standard evaluation so I'm in favor of adding grid.col, grid.lty, grid.lwd in tpar(). Maybe it would be useful to be able to pass them directly to tinyplot() as well?

[vincentarelbundock]

I'd vote yes for that(), which can have a ton of arguments, but I'd prefer limiting the number of arguments in tiny plot(), and the status quo already gives us an easy way to do this with grid=grid()