I have a store like below

```
Ext.define('Result', {
    extend: 'Ext.data.Model',
    fields: [
        { name: 'state', type: 'string', mapping:0 },
        { name: 'product', type: 'string', mapping:1 },
        { name: 'quantity', type: 'int', mapping:2 }
        { name: 'price', type: 'int', mapping:3 }
    ]
});
var store = Ext.create('Ext.data.ArrayStore', {
    model: 'Result',
    groupField: 'state',
    data: [
        ['MO','Product 1',50,40],
        ['MO','Product 2',75,50],
        ['MO','Product 3',25,60],
        ['MO','Product 4',125,70],
        ['CA','Product 1',50,50],
        ['CA','Product 2',100,40],
        ['WY','Product 1',250,40],
        ['WY','Product 2',25,50],
        ['WY','Product 3',125,86],
        ['WY','Product 4',175,83]
    ]
});

```

I want to calculate the average of quantity and price of every state group and show it into the grid .
I have seen in the sencha documentation, there is a function like [average](http://docs.sencha.com/extjs/4.2.2/#!/api/Ext.data.Store-method-average)(<http://docs.sencha.com/extjs/4.2.2/#!/api/Ext.data.Store-method-average>)
and I can Implement it like 

```
store.avarage('quantity', true);

```

but it only gives the average of only one field .

How and I calculate the average of multiple fields depending on the group and show it in a grid.