I use the calculation below to get the value in mega bits per second.
In your example for "wan1 sent" traffic the timestamps indicate you used the default resolution of 60.
result / (resolution * 125000) = Mbps
1562063 / (60 * 125000) = 0.21 Mbps
I seem to recall getting a lot of null values when I used the default resolution and timespan, like the data sometimes hasn't been saved to the meraki database, so I normally use 300 or higher for my resolution and couple that with the timespan value depending on how many data points you want returned.
I also noticed some really high values that seem to be out of range, maybe due to a counter reset, so you'll need to handle those exceptions as well.