Qt Custom Scatterplot
You should separate the actual data from the logic. Make your function capable of working on any QVectorq of QPairs (or pair of QVectors but that's not as normalized).

Variable names a,b,c,d don't make sense. Especially since a and b really ought to be input parameters.

You should be able to get both the min and max with one pass over the list. You'll probably have to write a custom mapreduce-style function.

For the circles in the QGraphicsScene: Are they supposed to have their center at the point in question, or the topleft of their bounding rect?