QPainter not drawing consistently

Question

I'm programming a GUI that draws graphs with Qt. My painter is showing some inconsistency: It only paints the graph about 50% of the times that I run the exact same binary after compilation. I do call QPainter's begin(), and I also made sure that the parameters I'm passing to the drawing functions such as drawEllipse() are initialized and have valid values when I call the function.

Below is the relevant code (note that the parameter painter has been initialized and begin() has been called prior to this function):

void GraphWidget::paintEvent(QPaintEvent *event) {
  QWidget::paintEvent(event);
  this->painter = new QPainter(this);
  painter->setRenderHint(QPainter::Antialiasing);
  // draw graph itself
  painter->translate(xOffset, yOffset);
  painter->scale(graphScale, graphScale);
  paintGraph(painter);
}

void GraphWidget::paintGraph() {
  if (this->graph) {
    // Iterate thought all edges and draw them
    for (Agnode_t *node = agfstnode(graph); node;
         node = agnxtnode(graph, node)) {
      for (Agedge_t *edge = agfstout(graph, node); edge;
           edge = agnxtout(graph, edge)) {
        drawEdge(edge);
      }
    }
    // Iterate through all nodes and draw them
    for (Agnode_t *node = agfstnode(graph); node;
         node = agnxtnode(graph, node)) {
      drawNode(node);
    }
  }
}

void GraphWidget::drawNode(Agnode_t *node) {

  ...

  //Height and width of node, in pixels.
  float scaleWidth = width * this->logicalDpiX();
  float scaleHeight = height * this->logicalDpiY();

  std::cout << "Drawing individual node. x = " << x << ". scaleWidth = " << scaleWidth << ". y = " << y << ". ScaleHeight = " << scaleHeight << "\n";
  //Actual node painting takes place here.
  painter->drawEllipse(x - scaleWidth / 2, y - scaleHeight / 2, scaleWidth, scaleHeight);

  ...

}

void GraphWidget::drawEdge(Agedge_t *edge) {
  // retrieve the position attribute and parse it
  float lastx, lasty, x, y;
  getNodePos(agtail(edge), lastx, lasty);
  auto spline_list = ED_spl(edge)->list;
  for (int i = 0; i < spline_list->size; i++) {
    x = spline_list->list[i].x;
    y = spline_list->list[i].y;
    painter->drawLine(lastx, lasty, x, y);
    lastx = x;
    lasty = y;
  }
  getNodePos(aghead(edge), x, y);
  painter->drawLine(lastx, lasty, x, y);
}

Answer 1

Found the problem. Calling painter->translate(xOffset, yOffset) was causing the painter issues because when I first opened the window, xOffset and yOffset were uninitialized so my guess is they took random values and the graph was being translated to some random spot where I couldn't see it. I just made sure to initialize the offset variables to 0 in the constructor and this fixed the problem.