Here's a complete example using XML from start to end:

<apex:page controller="FC_Chart">
    <apex:includeScript value="{!$Resource.FC}"/>
    
    <div id="FC_canvas">
    </div>

    <button onclick="goback()">Back One Year</button>
    <button onclick="goforward()">Forward One Year</button>
    
<script>
function updatechart() {
    a = FusionCharts("myChartId")
    a.setXMLData(document.getElementById('{!$Component.theForm.chartdata}').value)
}
</script>

<apex:form id="theForm">
<apex:inputHidden value="{!chartdata}" id="chartdata"/>
<apex:actionFunction action="{!goback}" reRender="theForm" name="goback" oncomplete="updatechart()"/>
<apex:actionFunction action="{!goforward}" reRender="theForm" name="goforward" oncomplete="updatechart()"/>
</apex:form>

<script>
a = new FusionCharts("{!$Resource.FC_SWF}","myChartId","400","300","0","1")
a.setXMLData(document.getElementById('{!$Component.theForm.chartdata}').value)
a.render("FC_canvas")
</script>    
</apex:page>

public class FC_Chart {
    date selecteddate = system.today();
    public void goback() {
        selecteddate = selecteddate.addmonths(-12);
    }
    public void goforward() {
        selecteddate = selecteddate.addmonths(12);
    }
    
    public string getchartdata() {
        map<integer,decimal> data = new map<integer,decimal>();
        string encodeddata = '<chart caption="Closed Sales for '+ String.valueOf(selecteddate.year()) +'">';
        date firstdate = selecteddate.addmonths(1-selecteddate.month()).tostartofmonth();
        date lastdate = selecteddate.addmonths(13-selecteddate.month()).tostartofmonth();
        for(integer i = 1; i < 13; i++)
            data.put(i,0);
        for(opportunity o:[select id,amount,closedate from opportunity where iswon = true and closedate >= :firstdate and closedate < :lastdate ]) {
            data.put(o.closedate.month(),o.amount+data.get(o.closedate.month()));
        }
        for(integer i = 1; i < 13; i++) {
            encodeddata += '<set label="' + datetime.newinstance(selecteddate.year(),i,1,1,1,1).format('MMM') + '" value="' + data.get(i) + '"/>';
        }
        encodeddata += '</chart>';
        return encodeddata;
    }
    
    public void setchartdata(string s) {
    
    }
}

Note that this method isn't recommended if there's any potential user input; you'd want to normally use the XMLWriter class to ensure proper escaping and such. This code is for demonstration purposes only, and does work in my developer organization as-is.

Note the use of the function getchartdata(), which posts data directly into a hidden field that we use to obtain the data in the AJAX request. This value is automatically encoded and decoded for us, so it is safely transported; the only problem we have is if a label translated to something that included a quotation mark. The solution would be EncodingUtil.URLEncode to make sure that the values do not exhibit this behavior.

You should be able to add the "export chart" function from here. Let me know if you need any further help.