[ksa]

Если я правильно понял...

const obj = { 
	weight: [
		{ "2021-05-01": 100},
		{ "2021-05-05": "99,9"},
		{ "2021-05-10": 99},
		{ "2021-05-15": "98,5"},
		{ "2021-05-20": 98 },
		{ "2021-05-25": 97.5 },
	],
	weist: [
		{ "2021-05-01": 100},
		{ "2021-05-05": "99,9"},
		{ "2021-05-10": 99},
		{ "2021-05-15": 98},
		{ "2021-05-20": 97},
		{ "2021-05-25": 96.5}
	]
}
let date = []
let total = [
	{name: 'weight', data: []},
	{name: 'weist', data: []}
]
for (let key in obj) {
	const ttl = total.find(e => e.name === key)
	for (let i = 0; i < obj[key].length; i++) {
		const a = Object.keys(obj[key][i])
		date[i] = a[0]
		ttl.data[i] = obj[key][i][a[0]]
	}
}
alert(JSON.stringify(date) + '\n' + JSON.stringify(total))