Feature/accessible sparklines

This covers accessible graphs for sparklines.

It's currently being merged into the graphs PR but we can change this to PR to master if that branch gets merged first.

We render tables for sparklines like so.


|      Time        | Number of Visits |
------------------|-----------------------
| 12:01:03 am |              5            |
| 12:02:03 am |              4            |
| 12:03:03 am |              0            |
| 12:04:03 am |              0            |

etc