I'm assuming you're referring to the following built-in report, which can be displayed in the chart report widget as well: "Chart: Computer model summary". To add the Custom20 field and keep the report compatible with the chart report widget, you'll need to combine the model name and custom field into a single expression, as the chart report widget only supports two columns. I've included a sample query below.
Select Top 1000000 LTrim(RTrim(Coalesce(tblAssetCustom.Manufacturer, N'') +
N' ' + Coalesce(tblAssetCustom.Model, N''))) + ' / Retirement: ' + Case
When tblAssetCustom.Custom20 Is Not Null And tblAssetCustom.Custom20 <>
'' Then tblAssetCustom.Custom20 Else 'unknown' End As Compmodel,
Count(tblAssets.AssetID) As Total
From tblAssets
Inner Join tblAssetCustom On tblAssets.AssetID = tblAssetCustom.AssetID
Where tblAssetCustom.State = 1 And tblAssets.Assettype = -1
Group By LTrim(RTrim(Coalesce(tblAssetCustom.Manufacturer, N'') + N' ' +
Coalesce(tblAssetCustom.Model, N''))) + ' / Retirement: ' + Case
When tblAssetCustom.Custom20 Is Not Null And tblAssetCustom.Custom20 <>
'' Then tblAssetCustom.Custom20 Else 'unknown' End
Order By Total Desc,
Compmodel