[accessibility] Activity graph does not have an accessible text version

[sandbergja]

Describe the bug A clear and concise description of what the bug is.

To Reproduce Steps to reproduce the behavior:

1. In Firefox, go to https://metacpan.org/pod/Test2
2. There is a graph showing activity on Test2 in the past 24 months. Right click it.
3. Select "Inspect Accessibility Properties"
4. This will open a tree view with the element type and the accessible name. Note that the accessible name for the image is an empty string.

Expected behavior

The SVG image has an alternative, so that screen reader users can also know what the activity has been like on a distribution.

Desktop (please complete the following information):

• OS: Windows
• Browser Firefox
• Version 127