Trying to view sheet after opening spreadsheet with openById() in google sheets - google-apps-script

Question

I have a Google Sheets that has D3.JS, i'm trying to use a specific sheet in the spreadsheet file, since that's where the table is located for the graph i'm trying to show. So far, the openById works but still is unable to locate the specific sheet in the spreadsheet.
function getChartData() {
var ss = SpreadsheetApp.openById("some id");
var sheet = ss.getActiveSheet();
var headings = sheet.getRange(1, 1, 1, sheet.getLastColumn()).getValues()[0]
.map(function(heading) {
return heading.toLowerCase();
});
...
}
EDIT: I've tried the following code changes below:
function getChartData() {
var ss = SpreadsheetApp.openById("some id");
var sheet = ss.getSheetByName("Tracker1");
// new code above
var headings = sheet.getRange(1, 1, 1, sheet.getLastColumn()).getValues()[0]
.map(function(heading) {
return heading.toLowerCase();
});
Logger.log(headings);
var values = sheet.getRange(3, 1, sheet.getLastRow()-2, sheet.getLastColumn()).getValues();
var data = [];
for (var i = 0; i < values.length; i++) {
var obj = {};
for (var j = 0; j < values[i].length; j++) {
obj[headings[j]] = values[i][j];
}
data.push(obj);
}
return data;
}
EDIT 10/2/18 11:00 PM : I'm still getting null, no error messages, no chart showing in app. I'm sure I'm doing something wrong, even without having getActiveSheet();