Adding Sparkline based on eval variable

[itssaad14]

I have a search which captures data from all the machines on the network and calculates OS Health of each machine (host). I am displaying it like this
OSHealth DeviceCount Percentage
5 288 35%
4 150 20%

I want to add a sparkline to show the trend of changing percentage and add it like another column. I have tried alot based on splunk docs but it always show as a straight line. I would really appreciate some help. My search is attached below

| stats count(host) AS DeviceCount by OSHealth
| eventstats sum(DeviceCount) AS SumDevice
| eval Percentage = round((DeviceCount/SumDevice)*100,1)
| stats sparkline avg(Percentage) as Trend by OSHealth DeviceCount
| table OSHealth DeviceCount Percentage

[richgalloway]

Sparklines require _time to work. However, the initial stats command is returning only the DeviceCount and OSHealth fields.