allow custom chart_attr

to make some ting like

    chart_attr = {
        'xAxis': '.rotateLabels(-25)',
        'margin': {'bottom': 90}
    }
    data = {
        'charttype': charttype,
        'chartdata': chartdata,
        'chartcontainer': chartcontainer,
        'extra': {
            'x_is_date': False,
            'x_axis_format': '',
            'tag_script_js': True,
            'jquery_on_ready': True,
            'chart_attr': chart_attr,
        },