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<?
$r = array();
$d = array();
$query = $mysqli->query("SELECT * FROM history WHERE id2='1'");
while ($set = $query->fetch_array()) {
  $r[] = $set['sum'];
  $d[] = '"'.date("d.m", $set['time']).'"';
  
}
$dt = implode(", ", $d);
$str = implode(", ", $r);
?>
var myArr = [<?=$str?>];
var Date = [<?=$dt?>];

стиль canvas игнорит поэтом ширину/высоту без style="" передавайте. Выгружать следует списком (больше одного в лимите)

<?
$r = array();
$d = array();
$query = $mysqli->query("SELECT * FROM history WHERE id2='1' ORDER BY id DESC LIMIT 10");
while ($set = $query->fetch_array()) {
  $r[] = $set['sum'];
  $d[] = '"'.date("d.m", $set['time']).'"';
  
}
$dt = implode(", ", $d);
$str = implode(", ", $r);
?>
<tr><td colspan="7">
<canvas id="buyers" width="500" height="200" class="tgr">
<script>
var myArr = [<?=$str?>];
var Date = [<?=$dt?>];
var buyers = document.getElementById('buyers').getContext('2d');
var buyerData = {
   labels : Date,
   datasets : [
      {
         fillColor : "rgba(172,194,132,0.4)",
         strokeColor : "#ACC26D",
         pointColor : "#fff",
         pointStrokeColor : "#9DB86D",
		 data: myArr
      }
   ]
}
</script>
<script>new Chart(buyers).Line(buyerData);</script></canvas>