Experienced unexpected and very unfortunate behavior in QCustomPlot that can be seen by plotting a Lissajous pattern. If you aren't familiar with such patterns, a simple example is to generate two variables, say x and y, that are both sinusoidal functions of another variable, often just a linearly growing time variable, t. That is plot y(t) vs x(t) for appropriate values of time.
If I do this, the x!y scatter points are plotted at the correct locations. Unfortunately, if I try to connect the time-sequential points with a connecting line, the resulting line is not at all what one would expect. It appears that internal to QCustomPlot, the x and y variables have been sorted by ascending values of x. Hence it is NOT points adjacent in time that are connected, but points by ascending x.
There is a flag, alreadySorted, for method setData for data that are guaranteed to be sorted, but I apparently can't trick the plot routine using that flag - the plot just doesn't work at all.
FYI, my plot strategy is a pretty basic QCustomPlot -> addGraph -> setData, etc.