OT? want to save web page image automatically

[Dave Stevens]

I have a web page that displays a chart of values that change once per
hour. It it uses chart.js under node.js on mint.

See it here: http://209.145.101.163/24chart.html

If I right click on the image I have an option (in Firefox) to save it
as a png. That works fine. What I'd like to do is save the image once
per hour without intervention. Does anyone know a scriptable method?

TIA

Dave