code: private void drawAxisBars(Graphics2D chartGraphics) {
      if (axisThickness > 0) {
        chartGraphics.setColor(axisColour);
  
        // Draw x-axis.
        int x = heatMapTL.x - axisThickness;
        int y = heatMapBR.y;
        int width = heatMapSize.width + axisThickness;
        int height = axisThickness;
        chartGraphics.fillRect(x, y, width, height);
  
        // Draw y-axis.
        x = heatMapTL.x - axisThickness;
        y = heatMapTL.y;
        width = axisThickness;
        height = heatMapSize.height;
        chartGraphics.fillRect(x, y, width, height);
      }
    }
docs: /*
  Draws the bars of the x-axis and y-axis.