Some instructions for those who are interested:

Freeware Irfanview instructions using Firefox browser.

Increase text size until color bar is at maximum size without extending past edges of map. Around 133% on my settings.

Scroll page so "Map" is barely touching top of screen. 

Go to full screen. Or screenshot will not get all of map and legend.

Right click the image > take screenshot. 
Have to do a "save visible" screenshot. Not a crop. 
Full page is unnecessary. And produces a much smaller final image size. 
Copy, not download. Paste into Irfanview.

Crop map and legend. Start crop at just above top of "Map". (Edit menu > crop selection).

Erase "Map". (Edit menu > show paint dialog > eraser). 

Text below to insert with example date. Use date from hovering over a map state.

COVID-19 death rates per 100,000.
USA. May 13, 2023.

On the map select a rectangle that barely touches the top of Maine. 
Or just click the map where the top left corner of that rectangle should be. Then:
Edit menu > insert text > Center the text > Arial. Regular. Black text. > Preview.

Change font size to fit the text at maximum size above the map.

Add border. (Image menu, add border).

Save as PNG. Compression level 9.
Use OptiPNG plugin; default level 2.
Do the above at the same time. Plugin lowers kilobytes significantly. Level 2 gets very slightly smaller kilobyte size then the much slower level 7! Level 2 takes a few seconds. Level 7 takes a minute. 

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