cesutherland
commented
11 years ago It looks to me like this is being calculated in units of the x and y axis. Because your x-axis is so small, and y-axis is so big, it's breaking. The calculation should be done in pixels (the scaled-to-your-screen x and y values) because that is what the mouse is using. I'll take a look.
In the mean time if you only have one series, set trackY to false: http://jsfiddle.net/cesutherland/yb2BS/
I can't figure out what it is about this data set that is causing it, but some mouse tracking points feel less "sensible" than others on this graph. [3,26551] is particularly difficult to hover over.
http://jsfiddle.net/jLzVb/1/