Improve Realtime Support

While using CairoPlot to plot data in realtime, problems were perceived on what concerns speed on long data arrays.
The problem seems to be caused by the excessive amount of horizontal labels (when the user doesn't input them), which are generated on the fly for each new point.
The idea is to provide an option for the user to choose what will be the interval between each label or, even, allow the user to remove them.