axisCommon = function () {
    return {
        axisLine: {
            lineStyle: {
                color: contrastColor
            }
        },
        axisTick: {
            lineStyle: {
                color: contrastColor
            }
        },
        axisLabel: {
            textStyle: {
                color: contrastColor
            }
        },
        splitLine: {
            lineStyle: {
                type: 'dashed',
                color: '#aaa'
            }
        },
        splitArea: {
            areaStyle: {
                color: contrastColor
            }
        }
    };
}
For big svg string, this method might be time consuming.

@param {string} svg xml string
@return {Object} xml root.