Chartjs-ChartJS only show me data when I zoom-in or zoom-out

Try calling your ajax method after initializing the chart object.

$(document).ready(function () {

    var ctx = document.getElementById("myChart");
    var myChart = new Chart(ctx, {
        type: 'horizontalBar',
        data: {
            labels: [],
            datasets: [{
                label: ['Mensual'],
                data: [],
                backgroundColor: [
                    'rgba(137, 107, 255, 0.3)'
                ],
                borderColor: [
                    'rgba(137, 107, 255, 1)',

                ],
                borderWidth: 1
            }
            ]
        },
        options: {
            scales: {
                yAxes: [{
                    ticks: {
                        beginAtZero: true
                    }
                }]
            }
        }
    });  
$.ajax({
        type: "POST", 
        url: "Graficas.asmx/obtenerGrafica",
        data: '',
        contentType: "application/json; charset=utf-8",
        dataType: "json", // dataType is json format
        success: function OnSuccess(response) {
            items = response.d;
            $.each(items, function (index, val) {
                myChart.data.labels.push(val.Nombre);
                myChart.data.datasets[0].data.push(val.Gastado);
                nmbre.push(val.Nombre);
                precio.push(val.Gastado);
            });
        },
        error: function onError(error) {
            console.log(error.d);
        },
    });         
});

Add two arrays before your ajax call:

var nomreArr= [];
var gastadoArr= [];    

Then populate them in the success function
and then assign them to the datasets

nomreArr.push(val.Nombre);
gastadoArr.push(val.Gastado);

myChart.data.datasets[0].data = gastadoArr;
myChart.data.datasets[0].labels= nomreArr;
myChart.update();

also your HTML should be like:

<canvas id="myChart" width="400" height="150"></canvas>

I had his same exact issue, and the only way I was able to fix it was by adding the following just before closing the document.ready block:

setTimeout(function() {
    myChart.update();
},1000);

It’s less than ideal but that was the only thing I could do to consistently get the chart to render — call the “update” method after a 1 second delay.