How to calculate RSI in Swift

To calculate the relative strength index (RSI) based on chart data in Swift, you would need to follow these steps:

1. Obtain the historical data for the security you are analyzing. This data should include the closing prices for each period (e.g. daily, weekly, etc.).
2. Calculate the difference between the current closing price and the previous closing price for each period.
3. Calculate the average gain and average loss for the last 14 periods. To do this, sum up the gains and losses from the previous step, and divide by 14.
4. Calculate the relative strength (RS) by dividing the average gain by the average loss.
5. Calculate the RSI by applying the formula: RSI = 100 - (100 / (1 + RS))
6. Finally, you can plot the RSI on a chart to visualize the data.

The code can be something like this:

func calculateRSI(closingPrices: [Double]) -> [Double] {
    var gains: [Double] = []
    var losses: [Double] = []
    var RSIs: [Double] = []
    
    for i in 1..<closingPrices.count {
        let change = closingPrices[i] - closingPrices[i-1]
        if change > 0 {
            gains.append(change)
            losses.append(0)
        } else {
            gains.append(0)
            losses.append(-change)
        }
    }
    
    for i in 14..<closingPrices.count {
        let avgGain = gains[(i-14)..<i].reduce(0, +) / 14
        let avgLoss = losses[(i-14)..<i].reduce(0, +) / 14
        let RS = avgGain / avgLoss
        let RSI = 100 - (100 / (1 + RS))
        RSIs.append(RSI)
    }
    
    return RSIs
}

Please note this is a sample code and it could be different based on your data structure and the way you handle it but it should give you a good idea on how to start.