There are no signals emitted from QAbstractSeries when the content of the series changes. So basically when you call addSeries(), the chart simply uses whatever data is there at the call, adds it to the plot, and then ignores any changes. QChart and QAbstractSeries are poorly-designed classes because they have no way for the series to tell the chart about changes, and no way for the chart to automatically update when a change is made to the series.

QChart is derived from QGraphicsItem, so calling QGraphicsItem::update() might work.