Generate simple image on a standalone Raspberrry Pi

[Roy Hann]

I am designing a mobile application to run on a Raspberry Pi 3 model B.
It will not have any Internet access. I need to generate a static image
consisting of a simple arc representing (say) a speedometer or a
pressure gauge. The image will need to be regenerated every 5 seconds.
The image must be displayed in a web browser (served by gunicorn
running on the Pi). I prefer it to be a PNG but that is only a
preference.

Plotly is amazing but big and slow. Pygal looked acceptable but
Internet access is required to render the SVG. 

Has anyone any suggestion for a lightweight solution? 

Roy