Overlays are useful when you want to call special attention to one of the series in a graph, particularly in a bar or column graph. You call attention to the series by defining it as an overlay. An overlay series is graphed as a line on top of the other series in the graph. To define a series as an overlay, do as follows:
To specify a column name to identify the series, specify this for the series:
"@overlay~t" + ColumnName
To use a label to identify the series, specify this for the series:
"@overlay~tSeriesLabel "
