The function you provided sets the label for a slice whether or not it's selected. (Make sure you've set the chart's
delegate correctly so this function is called.)
If you want something to change when a slice is selected, you should take a look at the method
sChart(_ chart: ShinobiChart, toggledSelectionFor dataPoint: SChartRadialDataPoint, in series: SChartRadialSeries, at pixelPoint: CGPoint).
The pie chart in the shinobicharts Samples app (which is written in Swift) uses this method, so it may help to take a look at that.
You can change the chart's background color using:
chart.backgroundColor = .clear
You may also need to change the plot area's background color as follows:
chart.plotAreaBackgroundColor = .clear