Feature Request: Alternating Background Color

This can be achieved in matplotlib using axvspan, but does not appear to be available in mplfinance. When plotting both the stock price and volume, it's difficult to match the two up, so the ability to switch shadings every other candle (from white to light grey) would improve the readability of the charts.

Start by reviewing mplfinance's plotting entry points and matplotlib's axvspan behavior to understand how candle positions map across price and volume panels. The work is done when alternating white and light-grey shading can be applied every other candle and remains aligned across both panels.

Written by the indexing model from the issue text.