use better names in `plot.py`

this MR goes from

let xs = seq 0 5 | each { 2 ** $in } | wrap x
let twice = $xs | insert measurement { 2 * $in.x } | insert error { 0.1 + 0.5 * $in.x }
let square = $xs | insert measurement { $in.x ** 2 } | insert error { 1 + 1.5 * $in.x }

python scripts/plot/plot.py --title title --x-label x --y-label y --fullscreen ([
    { group: "x ^ 2", items: $square },
    { group: "2 * x", items: $twice }
] | to json)

to

let xs = seq 0 5 | each { 2 ** $in }
let twice = $xs | wrap x | insert y { 2 * $in.x } | insert e { 0.1 + 0.5 * $in.x }
let square = $xs | wrap x | insert y { $in.x ** 2 } | insert e { 1 + 1.5 * $in.x }

python scripts/plot/plot.py --title title --x-label x --y-label y --fullscreen ([
    { name: "x ^ 2", points: $square },
    { name: "2 * x", points: $twice }
] | to json)

updates the "bench" README and adds type annotations to the plot.py script.

examples/benches/README.md

@@ -65,7 +65,7 @@ for graph in [
                 | where op == $graph.op
                 | rename --column { n: "x", mean: "y", stddev: "e" }
                 | group-by name --to-table
-                | rename --column { group: "name", items: "data" }
+                | rename --column { group: "name", items: "points" }
                 | to json
         )
     ]
@@ -95,7 +95,7 @@ python scripts/plot/plot.py ...[
             }}
             | rename --column { degree: "x", mean: "y", stddev: "e" }
             | group-by name --to-table
-            | rename --column { group: "name", items: "data" }
+            | rename --column { group: "name", items: "points" }
             | to json
     )
 ]
@@ -118,7 +118,7 @@ python scripts/plot/plot.py ...[
             | insert degree { get label | parse "degree {d}" | into record | get d | into int }
             | rename --column { degree: "x", mean: "y", stddev: "e" }
             | group-by name --to-table
-            | rename --column { group: "name", items: "data" }
+            | rename --column { group: "name", items: "points" }
             | to json
     )
 ]
@@ -145,7 +145,7 @@ python scripts/plot/plot.py --title "recoding with k = 4" (
         | where k == 4  # $k$ has a negligible influence on _recoding_
         | rename --column { bytes: "x", mean: "y", stddev: "e" }
         | group-by case --to-table
-        | rename --column { group: "name", items: "data" }
+        | rename --column { group: "name", items: "points" }
         | insert style {|it|
             let g = $it.name | parse "{c} / {s}" | into record | into int s
             let c = match $g.c {