Are you setting the axis yourself? If so, you may need to call [chart redrawChart] to get the axis to display properly.
As some background: December 31st 1969 is equivalent to "Day -1", as the Unix Epoch is January 1st 1970 (i.e. unix time zero).
To hide the axis values, you can set "chart.xAxis.style.majorTickStyle.showLabels = NO". You will need to call [chart redrawChart] after this too.