Just squares in "chart-image" .png

Hi all

I've been struggling with a strange problem generating a chart .png with chart-image node for a while now.
All text labels on all axes, values and the header are displayed as squares instead of readable text.

Like this:

There are some examples so I suspect the code is correct.
Line chart code:

Basic flow:

Anyone have any tips on how to display the text correctly?
Some missing node-red settings?


Additionally, the debug Output after the function line chart shows the correct (text) values:

Usually that indicates that the required font is missing

Where do I have to set the font?

presumably you can set it in the call to the charting library you are using.
Whatever you set it to has to be recognised by the browser you are using to view it - and installed on the computer where the browser is. Normally they should default down to something that is installed but in this instance it seems not.

Solved it! THX
Got the missing settings on my Ubuntu 18.04.3 LTS with sudo apt install fontconfig and then fc-cach -f -v

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