Add support for grids

[simonrw]

This support is basic so far, and only for continuous views for now.

The user can choose how many grid points they wish to use for each axis
(defaults to 3 each), and the colour of the grid lines (defaults to a
grey colour).

Additionally abstract two functions for drawing horizontal/vertical lines.

[milliams]

Looks good 😄 We probably don't want vertical grid lines for discrete views anyway so it makes sense to focus on continuous first.

Could you add a line to the CHANGELOG for this?

[simonrw]

Update changelog

[milliams]

I'm happy to merge this if you're ready?

[simonrw]

I think I'd like to have it at least implemented for CategoricalViews on the y axis. Hang on...

[simonrw]

There we go, horizontal grid lines for CategoricalViews

I've also dropped the grid's z ordering down so that plot points and lines show up above the grid. @milliams how do you feel about this? matplotlib renders the grid lines above the plots. I feel rendering the grid underneath looks nicer.

matplotlib

plotlib

[milliams]

I think I prefer your style with the grid lines in the background and the data points on top.

[simonrw]

Glad to hear it. Just updated the documentation and fetched the latest changes from master.

It's still probably not feature complete, but we can build smaller features on top of this. I think we should merge this as it is.

Some thoughts I have:

• configuring grid colour
• basing grid spacing on the plot range
• configuring what z-index the grid has
• overriding our defaults for horizontal vs both
• probably more...

[milliams]

You're right that there're a bunch more features that we need to add to this in the future. Feel free to open issues for some of those improvements or we can just refer back to this PR.