Correction, I'm not sure if you can use the chart name as an argument to chartojects.
But if it is the only chart in the sheet than you can pull it out of the sheet chartobjects collection using index 1, like this:
Worksheets("ACUITY Vs ISTEP GRAPH").ChartObjects _
(1).Chart.SeriesCollection(1).ApplyDataLabels _
Type:=xlDataLabelsShowValue, AutoText:=True, _
LegendKey:=False