Chart showing frequency of values

[solpyro]

I have the frontmatter field mood: 🙂 in my daily entries, and I want to build a pie chart to show the frequency of each emoji (or any unique string really).

My current attempt is

searchType: frontmatter
searchTarget: mood
dataset: mood
pie:
    label: mood
    data: mood

which renders the error No valid Y value found in notes. I'm sure I need to do some processing on the data to group and count the unique values, but I can't find how to do that in the examples and documentation.