Line chart does not showing continuous lines when data is loaded dynamically

Line chart is not showing when multiple lines are loaded into the graph.
The scenario is:
1: generate a chart with series1
2: Load a new series2 onto the chart
3. flow new set of data onto series1
Series one will be displayed as a new dot instead of showing as a line
The behavior is explained in this fiddle http://jsfiddle.net/xg06skg1/.