Add some explanatory text + diagrams

data-tidy.Rmd

@@ -228,6 +228,69 @@ billboard_tidy |>
   scale_y_reverse()
 ```
 
+### How does pivoting work?
+
+Now that you've seen what pivoting can do for you, it's worth taking a little time to gain some intuition for what's happening to the data.
+Let's make a very simple dataset to make it easier to see what's happening:
+
+```{r}
+df <- tribble(
+  ~var, ~col1, ~col2,
+   "A",     1,     2,
+   "B",     3,     4,
+   "C",     5,     6
+)
+```
+
+Here we'll say there are three variables `var` (already in a variable), `name` (the column names in the column names), and `value` (the cell values).
+So we can tidy it with:
+
+```{r}
+df |> 
+  pivot_longer(
+    cols = col1:col2,
+    names_to = "names",
+    values_to = "values"
+  )
+```
+
+How does this transformation take place?
+It's easier to see if we take it component by component.
+Columns that are already variables need to be repeated, once for each column in `cols`, as shown in Figure \@ref(fig:pivot-variables).
+
+```{r pivot-variables}
+#| echo: false
+#| out.width: ~
+#| fig.cap: > 
+#|   Columns that are already variables need to be repeated, once for
+#|   each column that is pivotted.
+knitr::include_graphics("diagrams/tidy-data/variables.png", dpi = 144)
+```
+
+The column names become values in a new variable, whose name is given by `names_to`, as shown in Figure \@ref(fig:pivot-names).
+They need to be repeated for each row in the original dataset.
+
+```{r pivot-names}
+#| echo: false
+#| out.width: ~
+#| fig.cap: >
+#|   The column names of pivoted columns become a new column.
+knitr::include_graphics("diagrams/tidy-data/column-names.png", dpi = 144)
+```
+
+The cell values also become values in a new variable, with name given by `values_to`.
+The are unwound row by row.
+Figure \@ref(fig:pivot-values) illustrates the process.
+
+```{r pivot-values}
+#| echo: false
+#| out.width: ~
+#| fig.cap: >
+#|   The number of values are preserved (not repeated), but unwound
+#|   row-by-row.
+knitr::include_graphics("diagrams/tidy-data/cell-values.png", dpi = 144)
+```
+
 ### Many variables in column names
 
 A more challenging situation occurs when you have multiple variables crammed into the column names.