Seems I've missed som formatting (the lint failure). Is there anything I need to do?

Thanks for the contribution @marcusmoller! On quick glance it looks like an issue with the linting tool rather than your change so no action needed.

I'll have closer review and merge within a couple days

Thanks again for this contribution

Could you share the use case for this format of output?

I have a few general comments:

1. If a new output format is being added, I think it should generally match a widely used data serialization standard rather than being something proprietary
2. If a new type of data is being output, I think it should support all existing output formats (i.e. JSON and CSV) unless there's some specific reason to exclude it

The use case is either a) just printing and reading the holdings summary without being inside a TUI or b) for easy parsing values for other terminal use-cases. Please see the readme for easy parsing using AWK.
I personally use it for adding my daily change to my window manager bar (i3bar with i3blocks) using the following:

[stock]
command=/home/marcus/PROGRAMMING/ticker/ticker print --summary | awk '/Day change/{print $3 " " $4}'
interval=60 # updated every min
color=#EBDBB2

As for 1) and 2) perhaps I could add two extra flags for printing in JSON and CSV format? Eg

$ ticker --config=./.ticker.yaml print --summary --json