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.