My chart lines are not in order

I have a graph and the result of joining all the lines should be as in the image. but currently they are not. what am I doing wrong?

https://i.stack.imgur.com/Bi7lq.jpg

https://jsfiddle.net/r2uaq27a/

var chart = c3.generate({
data: {
xs: {
data1: "data2"
},
columns: [
['data1', 2,3,4,7,8,9,8,7,3],
['data2', 1,2,3,4,5,4,3,2,1]
]

}
});

Start with the linked JSFiddle and reproduce the chart using the supplied c3.generate configuration. Compare the rendered line order with the linked image, focusing on the data1/data2 columns and xs mapping. Done means the chart connects the points in the intended order and the behavior is verified in the fiddle.

Written by the indexing model from the issue text.