I've spent hours trying to solve this. I've boiled it down to the following VF test page below. When I change the "Plan" selectList and there is no data, the pageBlockSection is not supposed to render since SalesInvestmentPlanData.HavePlan1Data and SalesInvestmentPlanData.HavePlan2Data are false. "UpdateDashboard" runs from the constructor, and also when the Plan selectList changes. I set HavePlan1Data and HavePlan2Data to false in the test code that runs in the controller shown below the VF. When the page loads, the pageBlockSection is not rendered, which is what I expect. When I change the Plan selectList, I get the error "Expected at least 1 rendered series component", implying that the pageBlockSection is rendering  but the Line Series are not rendering. Presume this is something obvious but just can't see why the pageBlock is rendered on the selectList change when HavePlan1Data and HavePlan2Data are hard-coded to false.  Greatly appreciate help on this.  PS I put this in a panelgrid because there are more charts in the original VF page. Thanks.

<apex:page StandardController="Investment_Plan__c" extensions="FinancialDashboardControllerTest" sidebar="False" LightningStyleSheets="True">
    
    <apex:pageBlock >
        <apex:form >
            <apex:pageBlockSection collapsible="False" columns="2">               
                <apex:selectList value="{!selectedOption}" size="1" label="Plan 1">
                    <apex:actionSupport event="onchange" action="{!UpdateDashboard}" rerender="idSelectedResourceScenario,idSelectedDate,idFinancialData"/>
                    <apex:selectOptions value="{!selectPlanOptions}"/>
                </apex:selectList>               
                <apex:selectList value="{!selectedResourceScenario}" size="1" label="Resource Scenario" id="idSelectedResourceScenario">
                    <apex:actionSupport event="onchange" action="{!UpdateDashboard}" reRender="idSelectedDate,idFinancialData"/>                    
                    <apex:selectOptions value="{!selectedResourceScenarioOptions}"/>
                </apex:selectList>               
                <apex:selectList value="{!selectedDate}" size="1" label="Projection Date" id="idSelectedDate">
                    <apex:actionSupport event="onchange"  rerender="idCharts"/>                    
                    <apex:selectOptions value="{!selectedDates}"/>
                </apex:selectList>               
            </apex:pageBlockSection>
        </apex:form>
    </apex:pageBlock>   
    <apex:pageBlock>
        <apex:pageBlockSection columns="1" id="idFinancialData" rendered="{!If(SalesInvestmentPlanData.HavePlan1Data || SalesInvestmentPlanData.HavePlan2Data,'True','False')}">
            <apex:messages/>
            <apex:panelGrid columns="1" id="idCharts">               
                <apex:outputPanel>                                        
                    <apex:chart height="300" width="510" data="{!SalesInvestmentPlanData.PlanData}">
                        <apex:legend position="bottom"/>
                        <apex:axis type="Numeric" position="left" fields="data1" minimum="0" maximum="{!SalesInvestmentPlanData.MaxPlanData}"/>   
                        <apex:axis type="Category" position="bottom" fields="name" >
                        </apex:axis>
                        <apex:lineSeries axis="left" xField="name" yField="data1" title="Plan 1"
                                         markerType="cross" markerSize="4" markerFill="#FF0000" rendered="{!If(SalesInvestmentPlanData.HavePlan1Data,'True','False')}"/>
                        <apex:lineSeries axis="left" xField="name" yField="data2" title="Plan 2"
                                         markerType="circle" markerSize="4" markerFill="#8E35EF" rendered="{!If(SalesInvestmentPlanData.HavePlan2Data,'True','False')}"/>           
                    </apex:chart>                                                        
                </apex:outputPanel>               
           </apex:panelGrid>
        </apex:pageBlockSection>
    </apex:pageBlock>   
</apex:page>

"Update Dashboard"
    public pageReference UpdateDashboard() {
        salesReturnData.HavePlan1Data = False;
        salesReturnData.HavePlan2Data = False;
        return null;
        }