In order to draw curved line graphs you need jquery.flot.curvedlines.js i.e the curved line graph plugin.
  FlotSeries serie = new FlotSeries
  {
    label = "Sample Curved Line Graph",
    data = graphData,
    curvedLines = new CurvedLineGraph { show = true }                 // This will denote that you want a curved Line graph
  };
  FlotOptions options = new FlotOptions
  {
    xaxis = new AxisOptions(),
    yaxis = new AxisOptions(),
    grid = new GridOptions { borderWidth = 1 },
    series = new SeriesOptions
    {
      curvedLines = new CurvedLineOptions { show = true }                       // This actitvates the curved lines plugin
    }
  };