QwtPlotBarChart with characters typed on the bars

I have a small data display program, which is presenting data in bar chart. Each bar represents the data from different source, so I need present the source of data with a letter on the top of each bar. Adding a source to bar legend below the chart is not enough, while the chart is updating constantly and user has to be able to read value and source easily. I checked the Distrowatch example, but I could not find how to define legend position. What is the easiest way to do this?

What exactly do you like to do - adding extra information to the legend, drawing something on top of the columns or changing the labels on the scales ?

Uwe