i did it
<chart>
<searchString>index=main sourcetype="Perfmon:Free Disk Space" counter="% Free Space" | chart avg(Value) as FreeSpacePercent by host |sort - FreeSpacePercent
| eval redCritical = if(FreeSpacePercent >= 85,FreeSpacePercent ,0)
| eval yellowWarning = if(FreeSpacePercent > 55 AND FreeSpacePercent <=84,FreeSpacePercent ,0)
| eval greenOK = if(FreeSpacePercent < 54,FreeSpacePercent ,0)
| table Datastore,redCritical,yellowWarning,greenOK</searchString>
<option name="charting.legend.labels">[redCritical,yellowWarning,greenOK]</option>
<option name="charting.seriesColors">[0xFF0000,0xFFFF00,0x00FF00]</option>
<option name="charting.primaryAxisTitle.text">FreeSpacePercent</option>
<option name="charting.chart.stacked">FreeSpacePercent</option>
<option name="charting.chart">column</option>
</chart>
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