Your chart has three distinct areas, the the UIView it is contained within, the canvas (where the chart & axis labels are rendered) and the plot area (aka GLView, which is where your data is rendered). Each of these internal views has the property 'clipsToBounds', so all you need to stop things like labels and annotations being cut off is to set this property on the relevant view to false:
_chart.ClipsToBounds = false;
_chart.Canvas.ClipsToBounds = false;
_chart.Canvas.GLView.ClipsToBounds = false;
From your picture, I cannot tell whether it is your chart's frame that is clipping your labels, but I suspect it probably is, so try _chart.ClipsToBounds = false first.