Add a guide for the AggregatorSheet (#2558)

Co-authored-by: Saul Pwanson <[email protected]>

Author: thejud

visidata/guides/AggregatorsSheet.md

@@ -0,0 +1,84 @@
+---
+sheet: AggregatorSheet
+---
+# Aggregations like sum, mean, and distinct
+
+Aggregators provide summary statistics for grouped rows.
+
+The current aggregators include:
+
+   min           smallest value in the group
+   max           largest value in the group
+   avg/mean      average value of the group
+   mode          most frequently appearing value in group
+   median        median value in the group
+   q3/q4/q5/q10  add quantile aggregators to group (e.g. q4 adds p25, p50, p75)
+   sum           total summation of all numbers in the group
+   distinct      number of distinct values in the group
+   count         number of values in the group
+   keymax        key of the row with the largest value in the group
+   list          gathers values in column into a list
+   stdev         standard deviation of values
+
+## View a one-off aggregation of a column
+
+- {help.commands.memo-aggregate} 
+
+## Create an aggregator column
+
+Aggregated columns appear in the **Frequency Table** and **Pivot Table** (grouped sheets).  Aggregated values will also appear at the bottom of their columns in the source sheet.
+
+- {help.commands.aggregate-col}
+
+Then aggregate the sheet with one of the grouping commands:
+
+- {help.commands.freq-col}
+- {help.commands.pivot}
+
+Aggregators can be viewed and modified on the **Columns Sheet** in the `aggregators` column.
+
+- {help.commands.columns-sheet}
+
+## The Describe Sheet
+
+To get a predefined set of summary statistics for every column in the sheet, use the **Describe Sheet* :
+
+- {help.commands.describe-all}
+
+## Examples
+
+Sample input sheet **sales**:
+
+   date        color  price
+   ----------  -----  -----
+   2024-09-01  R      30
+   2024-09-02  B      28
+   2024-09-03  R      100
+   2024-09-03  B      33
+   2024-09-03  B      99
+
+1. Move to the `price` column
+2. Set it to currency: [:keys]$[/key]
+3. Quickly show average price
+    - [:keys]z+[/] (`memo-aggregate`) then enter 'avg'
+4. Add an `sum` aggregator column:
+    - Press [:keys]+[/] (`aggregate-column`) then enter 'sum'
+5. Move to the date column [:keys]gh[/]
+6. Generate a **Frequency Table** by `date`
+    - [:keys]Shift-F[/] (`freq`)
+
+
+   date        count  price_sum
+   ----------  -----  ---------
+   2024-09-03  3      232.00
+   2024-09-01  1      30.00
+   2024-09-02  1      28.00
+
+## Creating new aggregator functions
+
+To add a new aggregator to compute the range of the grouped values (max - min), add the following to `.visidatarc`:
+
+[:code]vd.aggregator('range', lambda values: max(values) - min(values), 'range of values')[/]
+
+The `values` parameter is a list of typed values from the column, with the function returning the aggregated value.
+The new aggregator will now be available the next time VisiData is started.