To get the alternative text of an Excel chart, you need to look at the alternative text property of a shape.
I know I can use a workaround and access it in VBA but I have not been able to read the property in vanilla AS. Am I missing something?
To test, create a chart in Excel, right click on chart and select Format Chart Area…
tell application "Microsoft Excel"
set myShapes to shapes of active sheet
--{shape connector 1 of active sheet of application "Microsoft Excel", shape connector 2 of active sheet of application "Microsoft Excel"}
set shapeProperties to properties of (item 1 of myShapes)
(*
{class:shape connector, text frame:text frame of shape connector 1 of active sheet of application "Microsoft Excel", shape text frame:shape text frame of shape connector 1 of active sheet of application "Microsoft Excel", auto shape type:autoshape shape type unset, connection site count:4, connector:false, connector format:connector format of shape connector 1 of active sheet of application "Microsoft Excel", connector type:missing value, fill format:fill format of shape connector 1 of active sheet of application "Microsoft Excel", height:216.0, horizontal flip:false, left position:337.0, line format:line format of shape connector 1 of active sheet of application "Microsoft Excel", lock aspect ratio:false, name:"Chart 1", rotation:0.0, shadow format:shadow format of shape connector 1 of active sheet of application "Microsoft Excel", word art format:missing value, threeD format:threeD format of shape connector 1 of active sheet of application "Microsoft Excel", top:48.5, shape type:shape type chart, vertical flip:false, visible:true, width:324.0, z order position:1, hyperlink:missing value, black white mode:black and white mode automatic, shape on action:"", locked:true, top left cell:top left cell of shape connector 1 of active sheet of application "Microsoft Excel", bottom right cell:bottom right cell of shape connector 1 of active sheet of application "Microsoft Excel", placement:placement move and size, child:false, has chart:true, chart:chart of shape connector 1 of active sheet of application "Microsoft Excel", shape style:shape not a preset, background style:background not a preset, soft edge format:soft edge format of shape connector 1 of active sheet of application "Microsoft Excel", glow format:glow format of shape connector 1 of active sheet of application "Microsoft Excel", reflection format:reflection format of shape connector 1 of active sheet of application "Microsoft Excel", alternative text:"myText", character range:missing value}
*)
set altText to alternative text of (item 1 of myShapes)
-- missing value
end tell