title: Error filling print... java.lang.String cannot be cast to java.lang.Number
body: <p>I am trying sample report. I connected database successfully. </p>
  
  <p>I wrote simple query</p>
  
  <pre class="lang-sql prettyprint-override"><code>select country, population from dataset
  </code></pre>
  
  <pre class="lang-sql prettyprint-override"><code>desc dataset;
  
  
  | country    | varchar(25) | YES  |     | NULL    |       |
  
  | population | int(9)      | YES  |     | NULL    |       |
  </code></pre>
  
  <p>I have taken blank chart , In this I used XY Line chart. X-axis i have given <em>country</em> and y-axis <em>population</em>. When I click preview button, i am getting below error.</p>
  
  <pre><code>Error filling print... java.lang.String cannot be cast to java.lang.Number 
  java.lang.ClassCastException: java.lang.String cannot be cast to java.lang.Number
  at net.sf.jasperreports.charts.fill.JRFillXySeries.evaluate(JRFillXySeries.java:159)     
  at net.sf.jasperreports.charts.fill.JRFillXyDataset.customEvaluate(JRFillXyDataset.java:121)    
  at net.sf.jasperreports.engine.fill.JRFillElementDataset.evaluate(JRFillElementDataset.java:155)     
  at net.sf.jasperreports.engine.fill.JRCalculator.calculateVariables(JRCalculator.java:160)     
  at net.sf.jasperreports.engine.fill.JRVerticalFiller.fillDetail(JRVerticalFiller.java:765)     
  at net.sf.jasperreports.engine.fill.JRVerticalFiller.fillReportStart(JRVerticalFiller.java:288)     
  at net.sf.jasperreports.engine.fill.JRVerticalFiller.fillReport(JRVerticalFiller.java:151)     
  at net.sf.jasperreports.engine.fill.JRBaseFiller.fill(JRBaseFiller.java:909)     
  at net.sf.jasperreports.engine.fill.JRBaseFiller.fill(JRBaseFiller.java:822)     
  at net.sf.jasperreports.engine.fill.JRFiller.fill(JRFiller.java:61)     
  at net.sf.jasperreports.engine.JasperFillManager.fill(JasperFillManager.java:446)     
  at net.sf.jasperreports.engine.JasperFillManager.fill(JasperFillManager.java:276)     
  at net.sf.jasperreports.engine.JasperFillManager.fillReport(JasperFillManager.java:745)     
  at com.jaspersoft.ireport.designer.compiler.IReportCompiler.run(IReportCompiler.java:891)     
  at org.openide.util.RequestProcessor$Task.run(RequestProcessor.java:572)     
  at org.openide.util.RequestProcessor$Processor.run(RequestProcessor.java:997) 
  Print not filled. Try to use an EmptyDataSource...
  </code></pre>
  
  <p>Please help me.</p>