document x-axis customization

[anujgoyal]

Description

The x-axis has data labels on both the top and the bottom which looks a bit cluttered.

Expected behavior

It should be easy to customize the x-axis, please create documentation for.

1. removing/enabling the top label [DONE] coarse.time=TRUE||FALSE
2. removing/enabling the bottom label without extra whitespace remaining [WORKAROUND] format.labels <- ' '
3. removing/enabling tick marks for top x-axis
4. removing/enabling tick marks for bottom x-axis

Minimal, reproducible example

### basic
getSymbols("GM")
chart_Series(GM, subset='2012::2019')

### blanking bottom label, though the whitespace still persists
getSymbols("GM")
ct <- chart_theme()
ct$format.labels <- ' '
chart_Series(GM, subset='2012::2019', theme=ct)

getSymbols("GM")
ct <- chart_theme()
ct$coarse.time <- FALSE ###  removes the label above the x-axis
chart_Series(GM, subset='2012::2019', theme=ct)