Stackoverflow answer - http://stackoverflow.com/questions/41969347/array-to-string-conversion/41969600#41969600

file.php

<?php
include dirname(__FILE__) . '/../Database/dbcon.php';

header('Content-Type: text/html; charset=utf-8');

$inicio = $_REQUEST['inicio'];
$fin    = $_REQUEST['final'];
$sql    = "SELECT SUM(gastos.Cantidad) 'Cantidad', YEAR(gastos.Fecha) 'anio'
           FROM ciclo
           INNER JOIN gastos ON ciclo.Identificador = gastos.year_id
           WHERE ciclo.Ciclo BETWEEN $inicio AND $fin GROUP BY anio";

$resultado = $conn->query($sql) or die('fecha_fin');
$aux       = array();

while ($fila = mysqli_fetch_assoc($resultado)) {
    $aux[]  = (object)array(
        'year'  => $fila['anio'],
        'value' => $fila['Cantidad']
    );
}

echo "
<script>
new Morris.Line({
    element: 'graph_line',
    xkey: 'year',
    ykeys: ['value'],
    labels: ['Value'],
    hideHover: 'auto',
    lineColors: ['#26B99A', '#34495E', '#ACADAC', '#3498DB'],
    data: " . json_encode($aux) . "
});
</script>";