Looking at the code (http://jsfiddle.net/wybiral/3bdkp5c0/light/ http://jsfiddle.net/wybiral/3bdkp5c0/light/) it seems they are expanding the features to includ

Looking at the code (http://jsfiddle.net/wybiral/3bdkp5c0/light/ http://jsfiddle.net/wybiral/3bdkp5c0/light/) it seems they are expanding the features to include all second and third order terms (options.expansion = cubic), that's why linear regression does not come up with a straight line.