The arrow is showing the change in the last time period. I'll leave it as an exercise for the reader to modify the model to make the icon show the change over the time series (i.e., using the change between the first and last dates):
I've just released a new library called the Cell Icon Set Library. This contains a collection of pre-curated icon sets to be used as cell icons in tables. Icons are grouped into sets having a consistent theme. There are directional sets (like arrows and triangles), shape sets, indicator sets and ranking sets. Each icon set also uses a consistent color theme, with 5 color themes to select from. The functions also handle the logic of icon selection -- mapping from the values you want to depict to the icon to use. The two main functions, CellIconSetByLevel and CellIconSetByValue are used in place of the CellIcon function in your Computed Cell Format.
This image shows the available icon sets and color themes:
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