You will need to implement your own UITapGestureRecognizer, and attach it to the chart, then decide if the tap was on the left or right side.
The space given on either side of the chart is:
(axis.style.lineWidth + axis.titleLabel.width + axis.style.majorTickStyle.lineLength + axis.style.majorTickStyle.tickGap + largest label width)
If you have some features turned off (such as axis titles, or tickmarks), then you won't need to add these values.
Hope that helps!