Highcharts: Not plotting correctly with Y-Axis label formatter -

when use y-axis label formatter, points don't plot correctly. point "6.8" plotting above 6.8 line. also, y-axis ticks not equal intervals (7.8, 7.5, 7.3, 7.0, 6.8, 6.5). removing label formatter clears issue. know work-around can keep label formatter?

with label formatter: http://jsfiddle.net/3tkdt/1/

without label formatter: http://jsfiddle.net/3tkdt/2/

label formatter:

yaxis:  {   labels: {     formatter: function () {       return highcharts.numberformat(this.value, 1);     }   } } 

using numberformat function on y-axis labels wasn't idea. rounding tick marks , representing intervals incorrectly. removed label formatter.