As an introduction to what can be achieved with the form 'mixed' graph, this is an example taken from the distributions 'sample' directory to show an 'average' line contrasted against grouped bars to demonstrate 'variance' from the average.
There are in actual fact 3 types of graphs being used here but not all are being shown.
Area – is drawn at the bottom of the graph and is given a 'null' color value. The green and red data sets (bars) 'sit' on top of the area graph.
The line graph overlays the 3 preceding data sets.
The code to generate this is quite simple also, testimony to the designer of GD::Graph – a job very well done.