generate alt-text for graphs

[ashleyasmus]

see, e.g.:
"Graph showing viral load in Metro Plant wastewater as a blue line compared to metro-area COVID cases as shaded gray. Both measures trended upward beginning in late December and peaked in January, then a sharp decline until a smaller increase in April-May. Recent information shows a continuing slow downward trend. The number of cases generally follows wastewater data by 6-8 days. Last sample date June 6, 2022."

https://twitter.com/MetCouncilNews/status/1535298257651486721?s=20&t=MwBChEgQ3TeLp71qtos0jA