The standard method of exporting data from graphs to XLS, HTML, CSV files has the resolution of the selected view on the respective graph, not all data - due to performance issues. Please refer to the API article on how to retrieve data with REST requests.

In the following example, dashboards' JS scripting option is presented.

const DATA = {
         TIME: {START: 7, END: 8},
         DOWNLOAD_BTN: 9,
         GRAPH_ID: 1,
         GRAPH_SELECTION_REG: 11
};
const MAX_SLICES = 2999;
const TIMEOUT = 5e3;
const START_GRAPH_ID = 1; 
const END_GRAPH_ID = 2; 
 
let _i;
let preventMultiply = false;
let dataToSend = {};
let promises = [];
 
App.on('register:newValue', triggerUponRegChange);
 
function triggerUponRegChange(newValuesArray) {
    let ifButtonPressed = newValuesArray.filter(readVal => 
                                                    parseInt(readVal.regId) === DATA.DOWNLOAD_BTN 
                                                    && parseInt(readVal.value) > 0).length > 0;
    if (ifButtonPressed && !preventMultiply) {
            let selectedGraph = new Register(DATA.GRAPH_SELECTION_REG).getValue();
            selectedGraph = parseInt(selectedGraph.replace(/^.*\((\d)\).*$/, (match, p1) => p1));
            DATA.GRAPH_ID = (selectedGraph >= START_GRAPH_ID && selectedGraph <= END_GRAPH_ID) ? selectedGraph : 1; 
 
            Promise.all(makeApiCall()).then(exportDataAsCSVToBrowser).catch(e => console.log(e));
    }
}
 
function getSlicesFromTS() {
    let paramsSet = [];
    calcSlices(parseInt(new Register(DATA.TIME.START).getValue()),
               parseInt(new Register(DATA.TIME.END).getValue()));
 
    function calcSlices(s, e)  {
        if (s > e) { 
            console.log('startTS >= endTS') ; return ;  
        }
        if (e - s <= MAX_SLICES)  
            paramsSet.push({start:s, end: e, slices: e - s + 1});
        else {
            let newEnd = s + MAX_SLICES - 1;
            paramsSet.push({start: s, end: newEnd, slices: MAX_SLICES});
            calcSlices(newEnd + 1, e);
        }
    }
    return paramsSet;
}
 
function makeApiCall() {
    let slicesParams = getSlicesFromTS();
    dataToSend = {};
    promises = [];
    preventMultiply = true;
    setTimeout(function(){clearTimeout(_i); preventMultiply = false;}, TIMEOUT);
 
    slicesParams.forEach(function(paramsSet, i) {
        let promise = new Promise(function(resolve, reject) {
            $.ajax({
                url: "/api/graph-data/" + DATA.GRAPH_ID,
                method: "GET",
                headers: {
                    "X-WH-START": paramsSet.start,
                    "X-WH-END": paramsSet.end,
                    "X-WH-SLICES": paramsSet.slices,
                    "X-WH-REGISTERS": DATA.GRAPH_ID
                },
                dataType: 'json',
                context: this,
                success: function (response) {
                    dataToSend[i] = response;
                    resolve();
                },
                error: function (result, text, error) {
                    reject(text);
                }
            });
        });
        promises.push(promise);
    });
 
    return promises;
}
 
function exportDataAsCSVToBrowser() {
    let response = [];
    Object.values(dataToSend).forEach(dataSet => {response = response.concat(dataSet)});
    if (response.length > 0) {
        let rep = [];
        let header = ['Time'];
        let regs = WebHMI.graphs[DATA.GRAPH_ID].regs;
        regs.forEach(function(reg) {
            let vv = reg.title.replace("\"", "\"\"");
            header.push('"' + vv + " - Min" + '"');
            header.push('"' + vv + " - Avg" + '"');
            header.push('"' + vv + " - Max" + '"');
        });
        rep.push(header);
        response.forEach(function(dataSet) {
            let row = [moment(dataSet.x).format("YYYY-MM-DD HH:mm:ss")];
            regs.forEach(reg => dataSet[reg.id].split(';').forEach(value => {row.push(value)}));
            rep.push(row);
        });
 
        if (window.navigator.msSaveBlob) { // IE
            let blobObject = new Blob([rep.join("\r\n")]);
            window.navigator.msSaveBlob(blobObject, WebHMI.graphs[DATA.GRAPH_ID].title + '.csv');
        } else {
            let a = document.createElement('a');
            a.href = 'data:attachment/csv,' + encodeURIComponent(rep.join("\r\n"));
            a.target = '_blank';
            a.download = WebHMI.graphs[DATA.GRAPH_ID].title + '.csv';
            a.style.visibility = "hidden";
            document.body.appendChild(a);
            a.click();
        }
    }
}