Chart not displayed if X and Y axes are Numeric

[mycracks]

Hello everybody,
in my company we use ExtJS 4.0.7 (at the moment my boss doesn't want to upgrade to 4.1).
I have a problem with a line chart configured with both X and Y axes set to 'Numeric' and not displaying anything.
I'm sure that the data gets loaded from the server, because I've done a console.log of the store and I can see that they are there.
If I set the X axis to 'Category' I can see all the data, but I haven't got the same controls over the display as I get with 'Numeric' type, resulting in a mess of displayed labels.
My question is: how could I get the chart to work with both the axes set to 'Numeric'?
Thank you for your support.

Here is the code I use for the chart:

Ext.define('Project.view.stats.LineChart' ,{
    extend: 'Ext.chart.Chart',
    alias : 'widget.mylinechart',
    height: 220,
    width: 450,
    animate: true,
    insetPadding: 20,

    initComponent : function() {
        this.store = Ext.create('Project.store.Chartdatas');

        this.axes = [
            {
                type: 'Numeric',
                decimals: 0,
                minimum: 0,
                fields: ['timestamp'],
                position: 'bottom',
                grid: true,
                title: "Date",
                label: {
                    renderer: function(value) {
                        var date = new Date(value * 1000);
                        var label = Ext.Date.format(date, "j M") + "\n" +
                                    Ext.Date.format(date, "H:i");
                        return label;
                    }
                }
            },
            {
                type: 'Numeric',
                decimals: 0,
                minimum: 0,
                maximum: 10,
                fields: ['value'],
                position: 'left',
                grid: true,
                title: "Value"
            }
        ];

        this.series = [
            {
              type: 'line',
              xField: 'timestamp',
              yField: 'value'
            }
        ];

        this.callParent();
    }
});

This is my model:

Ext.define('Project.model.Chartdata', {
    extend : 'Ext.data.Model',
    fields : [
        {
            name : 'timestamp',
            type : 'int'
        },
        {
            name : 'value',
            type : 'double'
        }
    ],
    proxy : Ext.create('Project.proxy.DefaultProxy', {
        api : {
            read : '/get_chart_data'
        }
    })
});

And this is my Store:

Ext.define('Project.store.Chartdatas', {
    extend: 'Ext.data.Store',
    model: 'Project.model.Chartdata'
});

Everything is then inserted into a panel, which loads the store and displays the chart.

[mycracks]

You can get a feel for exactly what is my problem by copying and pasting this code (which reproduces my problem) here: http://try.sencha.com/extjs/4.0.7/do....1/viewer.html

/*global Ext:false */
Ext.onReady(function () {
    var store = Ext.create('Ext.data.JsonStore', {
        fields: ['xValue', 'yValue'],
        data: [
          {'xValue': 10,  'yValue': 2},
          {'xValue': 15,  'yValue': 7}, 
          {'xValue': 18,  'yValue': 22}, 
          {'xValue': 23,  'yValue': 4}, 
          {'xValue': 25,  'yValue': 13},
          {'xValue': 32,  'yValue': 0},
          {'xValue': 40,  'yValue': 1}
        ]
    });

    Ext.create('Ext.chart.Chart', {
        renderTo: Ext.getBody(),
        width: 500,
        height: 300,
        store: store,
        axes: [{
            type: 'Numeric',
            position: 'left',
            fields: ['yValue'],
            title: 'Y Axis',
            grid: true,
            minimum: 0,
            adjustMinimumByMajorUnit: 0
        }, {
            type: 'Numeric',
            position: 'bottom',
            fields: ['xValue'],
            title: 'X Axis',
            grid: true,
            label: {
                rotate: {
                    degrees: 315
                }
            }
        }],
        series: [{
            type: 'line',
            highlight: false,
            axis: 'left',
            xField: 'xValue',
            yField: 'yValue'
        }]
    });
});

and you'll notice that when you set X axis to 'Category' everything works, but when you set it back to 'Numeric' you will not see anything.