Edit in JSFiddle

var data = {
    "graph": {
        "datasequences": [{
            "datapoints": [{
                "title":
                    "86",
                    "value":
                    "86000"
            }, {
                "title":
                    "87",
                    "value":
                    "87000"
            }, {
                "title":
                    "88",
                    "value":
                    "88000"
            }, {
                "title":
                    "89",
                    "value":
                    "89000"
            }, {
                "title":
                    "90",
                    "value":
                    "90000"
            }, {
                "title":
                    "91",
                    "value":
                    "91000"
            }, {
                "title":
                    "92",
                    "value":
                    "92000"
            }, {
                "title":
                    "93",
                    "value":
                    "93000"
            }, {
                "title":
                    "94",
                    "value":
                    "94000"
            }, {
                "title":
                    "95",
                    "value":
                    "95000"
            }, {
                "title":
                    "96",
                    "value":
                    "96000"
            }, {
                "title":
                    "97",
                    "value":
                    "97000"
            }],
                "title":
                "Downloads"
        }],
            "refreshEveryNSeconds": 900,
            "title":
            "ATP"
    }
}

var episodes = data.graph.datasequences[0].datapoints;
var count = episodes.length;
var sum = 0;

for (var i=0; i < count; ++i) {
    sum += parseInt(episodes[i].value);
}
var lifetime = (sum / count).toFixed(1);

sum = 0;

for (var i=0; i < 10; ++i) {
    var itemNumber = count - i - 1;
    sum += parseInt(episodes[itemNumber].value);
}
var last10 = (sum / 10).toFixed(1);

$('#last10').html(last10);
$("#lifetime").html(lifetime);

Last 10: <span id="last10"></span><br />
Lifetime: <span id="lifetime"></span><br />