Tracking issue for grids

[simonrw]

This issue tracks the features we want to add to showing grids.

Checklist

• [x] Basic grid support (#23)
• [ ] configuring grid colour
• [ ] basing grid spacing on the plot range
• [ ] configuring what z-index the grid has
• [ ] overriding our defaults for horizontal vs both
• [ ] logarithmic grids
• [ ] support the text output