Edit in JSFiddle

var canvas = document.getElementById('inqstatsChart'),
context = canvas.getContext('2d');
window.addEventListener('resize', resizeCanvas, false);


function resizeCanvas() {
		canvas.width = window.innerWidth - 80;
		canvas.height = window.innerHeight - 70;
		getData("internetusers_percent", 1995, 2014);
}

var colorArray = ["51, 102, 255", "51, 153, 51", "237, 182, 17", "163, 29, 121", "222, 35, 35"];


function getData(datakey, startyear, endyear) {
	$.ajax({
		url: 'http://inqstatsapi.inqubu.com?api_key=7d39fbc067d21a0f&countries=at,de,us&data='+datakey+'&years='+startyear+':'+endyear+'&addRegion=true&lang=de',
		complete: function(response) {
			countries = JSON.parse(response.responseText);
			var datasets = []
			var number = 0;
			countries.forEach(function(country) {
				var dataarray = country[datakey];
				var data = [];
				for(var i=startyear;i<=endyear;i++) {	
					var set = false;				
					dataarray.forEach(function(datapoint) {
						if(i == datapoint.year) {
							data.push(parseFloat(datapoint.data));
							set = true;
						}
					});
					if(set == false) {
						data.push(null);
					}
				}
				datasets.push({"label":country.countryName,"borderColor":"rgba("+colorArray[number]+", 1)","backgroundColor":"rgba("+colorArray[number]+", 0)","data":data});
				number++;
			});
			drawChart(datasets, startyear, endyear)
        }, error: function(e) {
            alert("Data could not be loaded from URI\n" + e.statusText);
        },
	});
	return false;
}

function drawChart(datasets, startyear, endyear) {
var ctx = document.getElementById("inqstatsChart").getContext("2d");
var labels = [];
for(var i=startyear;i<=endyear;i++) {	
	labels.push(i);
}
var chart = new Chart(ctx, {
    type: 'line',
    data: {
        labels: labels,
        datasets: datasets
    },
    options: {
        scales: {
            yAxes: [{
                scaleLabel: {
					display: true,
					labelString: 'Internetnutzer (in % der Gesamtbevölkerung)'
				}
            }]
        }
    }
});
}

resizeCanvas();
<div>
    <canvas id="inqstatsChart"></canvas>
</div>

              
            
          
            
              

External resources loaded into this fiddle: