JSFiddle - React, Tailwind, and code Playground

JavaScript

function mergeJson(target) {
    for (var argi = 1; argi < arguments.length; argi++) {
        var source = arguments[argi];
        for (var key in source) {
            if (!(key in target)) {
                target[key] = [];
            }
            for (var i = 0; i < source[key].length; i++) {
                target[key].push(source[key][i]);
            }
        }
    }
    return target;
}

var json0 = {
    "1948": [
        {
            "Seattle": {
                "city": "Seattle",
                "data": {
                    "avg_dew_point": "34",
                    "avg_gust_wind": "",
                    "avg_max_temp": "45",
                    "avg_min_temp": "35",
                    "avg_precipitation": "0.00",
                    "avg_sea_level_pressure": "30.22",
                    "avg_temp": "40",
                    "avg_wind": "8"
                },
                "month": "1",
                "state": "WA"
            }
        },
        {
            "Chicago": {
                "city": "Chicago",
                "data": {
                    "avg_dew_point": "10",
                    "avg_gust_wind": "",
                    "avg_max_temp": "25",
                    "avg_min_temp": "11",
                    "avg_precipitation": "0.00",
                    "avg_sea_level_pressure": "30.17",
                    "avg_temp": "18",
                    "avg_wind": "12"
                },
                "month": "1",
                "state": "IL"
            }
        }
    ],
    "1949": [
        {
            "Houston": {
                "city": "Houston",
                "data": {
                    "avg_dew_point": "45",
                    "avg_gust_wind": "",
                    "avg_max_temp": "61",
                    "avg_min_temp": "44",
                    "avg_precipitation": "0.00",
                    "avg_sea_level_pressure": "30.15",
                    "avg_temp": "53",
                    "avg_wind": "12"
         ...