ureech,
<script>
let ar = [{"sum":"40.00","title":"Еда"},{"sum":"40.00","title":"Еда"},{"sum":"100.00","title":"Акции"},{"sum":"1000.00","title":"Зарплата"},{"sum":"100.00","title":"Акции"}]
let keys = {}, out = [];
ar.forEach(({title: name, sum : data}) => name in keys ? keys[name].push(data) : (data = keys[name]=[data], out.push({name, data})));
document.write(`<pre>${JSON.stringify(out, "", 1)}</pre>`)
</script>