charts improvements (a11y among others)

[graphieros]

This issue lists all charts a11y improvements to be done.
It is not exhaustive, and can be completed iteratively, and include other types of chart improvements or implementations.

Improve series recognition for users with achromatopsia

• bar charts - add background patterns

Improve chart navigability for keyboard users

• update vue-data-ui components with keyboard navigation in the chart area & legends + customisable visual hints

Improve chart data access

• update vue-data-ui components with hidden data tables & hints
• access to visual data tables when possible

various chart options

• quadrant chart on the compare page

[graphieros]

@knowler, I'm pasting here the second point you made in your comment on #2193:

Provide a visual of the data table since that can be valuable.

I think there are two different cases, on which I would love to have your thoughts:

1. large charts (downloads in the modal and the compare page, versions modal, bar charts in the compare page)

a. we could consider having a toggle (or tabs) to access a visual data table when the chart is constrained by a modal or by a small grid (like the bar charts in the compare page). Would the built-in sr data table be necessary in this case, since the data table would have to be accessed through an extra step ?
b. in the compare page, we could display the data table under the chart itself, without a toggle. In this case, would the built-in sr data table be redundant ? If so, an update would be necessary in vue-data-ui to add a flag to allow disabling this table from the configuration.

2. small charts (downloads sparkline)

displaying a data table for the weekly downloads sparkline chart of the package page (with 52 rows) is obviously not desirable, leading to a similar solution as 1a (adding a button to show a data table in a modal ?).