function onOpen() {
    SpreadsheetApp.getUi() // Or DocumentApp or FormApp.
        .createMenu('Zusatzfunktionen')
        .addItem('SankeyDiagram', 'openDialog')
        .addToUi();
}

function getSpreadsheetData() {
    // ID of your Document, take from URL
    var ssID = "",
        // which Sheet? [0] is the first and so on...
        sheet = SpreadsheetApp.openById(ssID).getSheets()[0],
        data = sheet.getDataRange().getValues();
    return data;
}

function openDialog() {
    var html = HtmlService.createHtmlOutputFromFile('index')
        .setHeight(300)
        .setWidth(1000);
    SpreadsheetApp.getUi() // Or DocumentApp or FormApp.
        .showModalDialog(html, 'Sankey Diagram');
}