When "Exporting Data" from a multi-history chart there are many nan's in the data.
I/A Series G3 (Niagara)
The timestamp in the history data has a millisecond component that is not shown on the data displays. When the Export data is displayed history values are provided with the timestamp. If the timestamps for all the histories samples are exactly the same (to the millisecond) the data value is displayed, if not "nan" is displayed.
This condition occurs only when the "rollup" for the chart is left at the default of none. Changing the "rollup" to a value other than none will bring the data into order. Setting the rollup to the sample interval of the histories gives the best results in the Export data.