Hi , 


This is currently not available, Vote for the Idea:

https://success.salesforce.com/ideaView?id=08730000000jvE4AAI


Try this one: 

<mx:BubbleChart
            id="mybubblechart"
            showDataTips="true"
            >
            <mx:horizontalAxis>
                <mx:CategoryAxis   title="Opportunity Stage"  categoryField="StageName" dataProvider="{opps}"/>   
            </mx:horizontalAxis>
           
            <mx:verticalAxis>
                <mx:CategoryAxis  title="User Probability" categoryField="User_Probability__c" dataProvider="{opps}"/>
            </mx:verticalAxis>
           
            <mx:series>               
                <mx:BubbleSeries
                    xField="StageName"
                    yField="User_Probability__c"
                    radiusField="Amount_Num__c"
                    fill="{sc1}"
                    stroke="{stroke1}"
                    dataProvider="{opps}"/>
            </mx:series>               
        </mx:BubbleChart>

http://funwithsfdc.blogspot.in/2010/10/bubble-chart.html

Or


http://code.google.com/p/visualforce-components/source/browse

look under trunk/bubblePlot


Or 

https://developer.salesforce.com/forums/ForumsMain?id=906F000000094KYIAY

https://developers.google.com/chart/interactive/docs/gallery/bubblechart


Regards,
Ashish