The graph drawing uses the [Braille Patterns](https://en.wikipedia.org/wiki/Braille_Patterns) Unicode block to simulate filled regions in line graphs, but if the font that your terminal emulator is using includes the unused dots in the characters as open holes, the result will look something like this:
When some package requires `ttf-font`, this package usually gets chosen. However, the fonts are kind of ass, so as long as you have another package that provides `ttf-font`, you're ok to remove `gnu-free-fonts`
Start by identifying a font that you want to use, and covers the braille characters, then set that as a default or fallback font per [these instructions from the Arch Wiki](https://wiki.archlinux.org/title/Font_configuration#Set_default_or_fallback_fonts)
