You'll need first to create a variability/gauge chart, then select the "show box plots" option to add the boxes. You should then find that the means are shown automatically, but if they're not, click on the small red triangle to the left of the "Variability Gauge" header and select "Show cell means". To join them up as well, check "Connect cell means".
The following script should create a set of box plots on a dummy data file and join the means as described above:
dt1 = New Table( "Data Table",
add rows( 50 ),
New Column( "Group", character ),
New Column( "Data", numeric )
);
For( i = 1, i <= N Row( dt1 ), i++,
Column( dt1, "Group" )[i] = Choose( Random Integer( 1, 5 ),
"A",
"B",
"C",
"D",
"E"
);
Column( dt1, "Data" )[i] = Random Uniform( 0, 100 );
);
Variability Chart(
Y( :Data ),
X( :Group ),
Analysis Type( Name( "Box Plot" ) ),
Connect Cell Means( 1 ),
Std Dev Chart( 1 ),
Show Box Plots( 1 )
);