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.

scripts/plot/plot.py

@@ -4,6 +4,29 @@ import sys
 import matplotlib.pyplot as plt
 import argparse
 
+from typing import List, TypedDict
+
+# all fields of a `Measurement` should have the same length
+class Measurement(TypedDict):
+    x: List[float]
+    y: List[float]
+    e: List[float]
+
+class LineStyle(TypedDict):
+    marker: str
+    type: str
+    width: int
+
+class Style(TypedDict):
+    color: str
+    line: LineStyle
+    alpha: float
+
+class Graph(TypedDict):
+    name: str
+    data: Measurement
+    style: Style
+
 HELP = """## Example
 ```nuon
 [
@@ -55,7 +78,7 @@ default values have been chosen:
 
 # see [`HELP`]
 def plot(
-    graphs,
+    graphs: List[Graph],
     title: str,
     x_label: str,
     y_label: str,