查询PHPLOT中的图形

I'm trying to do one graphic in phpplot, I want to group by year and month all the register users in that time and show in a graphic that results.

In database is like that:

id  | utilizador | registado
----+------------+--------------------
1   |   crisutu  | 2014-05-04 10:41:53
2   |   blabla   | 2014-05-15 04:45:24
3   |   hello    | 2014-06-15 04:45:24

I want to transform in the graphic like MAY 2014 - 2 register users JUNE 2014 - 1 register user

<?php
#Incluimos a biblioteca
require("phplot-6.1.0/phplot.php");



#Definimos o objeto para inicar a "montagem" do gráfico
#Também delimitamos uma altura e largura do gráfico
$grafico = new PHPlot(750,600);

#Indicamos o formato de imagem a ser usado
$grafico->SetFileFormat("png");

#Indicamos o títul do gráfico e o título dos dados no eixo X e Y do mesmo
$grafico->SetTitle("NSMF");
$grafico->SetXTitle("Meses");
$grafico->SetYTitle("Utilizadores");


function getValues(){
  $db = new mysqli("xxxxxxxxx", "xxxxxxxxxxxx", "xxxxxxxxxxxxx", "xxxxxxxxx");
  $db->set_charset("utf8");
  $query = $db->query("SELECT COUNT(id),registado FROM utilizadores GROUP BY YEAR(registado), MONTH(registado) ");

  while($obj = $query->fetch_object()){

  $values[] = array($obj->id, $obj->id );

  }
  return $values;
  }

$dados = getValues();

$grafico->SetDataValues($dados);

#Exibimos o gráfico
$grafico->DrawGraph();
?>

Can you help me resolve the problem? Tell me what is wrong?

So here are three mistakes you made in your code:

Accessing wrong property of object

I do believe that you are trying to access the property id of fetched object, but the property is actually COUNT(id). You may consider using alias in the query, eg:

SELECT COUNT(id) as id, registrado ...

Wrong data format

By default PHPlot::setDataValues expect following format:

array(
    array('key1', 'value11', 'value12'),
    array('key2', 'value21', 'value22'),
    array('key3', 'value31', 'value32'),
)

in your case keys are months so the first value of every array must be appropriate month, so write $values[] = array($obj->registado, $obj->id );, instead of $values[] = array($obj->id, $obj->id );

Date formating

Finally for your date to appear in desired format use php function date (http://php.net/manual/en/function.date.php) when assigning values to $value. So use something like this:

$values[] = array(date("%F %Y", $obj->registado), $obj->id);