Using overlays

It is often useful to call special attention to one of the series in a graph, particularly in a bar or column graph. You can do that by defining the series 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, define it as follows:

If specifying a column name to identify the series, specify this for the series:
```
"@overlay~t" + ColumnName
```
If using a label to identify the series, specify this for the series:
```
"@overlay~tSeriesLabel "
```