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The "axisPositionValue" is in data terms on the other axis.
For example, if you had a X-axis with the data range 0 - 10 and a Y-axis with the data range 0 - 100. To get both axes to be in the centre of the chart set the X-axis' "axisPositionValue" & the Y-axis' "axisPositionValue" to 5 like so:
chart.xAxis.axisPositionValue = @50;
chart.yAxis.axisPositionValue = @5;
Let me know if you have any questions.