Unfortunately, the i3 cannot display the charging rate directly. It does display the estimated time of charging completion, but because the charging rate tapers off as the battery pack's state of charge (SoC) approaches 100%, and because this completion time estimate is typically pessimistic, one could deduce only the average charging rate. Some public EVSE's display the charging rate.
The most accurate way to calculate the current charging rate would be to divide the change in SoC by the time and assume that with charging losses, the amount of electricity needed to fill an empty battery pack is ~20 kWh.
2014 BMW i3 Arravani Grey, Giga World, Tech + Driving Assist, Parking Assist, DC Fast Charging, JuiceBox EVSE