charts a11y improvements

[graphieros]

This issue lists all charts a11y improvements to be done.
It is not exhaustive, and can be completed iteratively.

Improve series recognition for users with achromatopsia

• bar charts - add background patterns
• line charts (multiple series) - add series names above or below the last datapoint value

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

[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 ?).