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
}
};