Tested with python -m pyqtgraph.examples

--> GraphicsItems --> Scatter Plot

Changed:
g.ScatterPlotItem(size=10, pen=pg.mkPen(None), brush=pg.mkBrush(255, 255, 255, 120))
to:
g.ScatterPlotItem(size=10, pen=pg.mkPen(None), brush=pg.mkBrush(255, 255, 255, 120), hoverable=True)
or:
g.ScatterPlotItem(size=10, pen=pg.mkPen(None), brush=pg.mkBrush(255, 255, 255, 120), hoverable=True, tip='x: {x:.3g}\ny: {y:.3g}\ndata={data}'.format)

Expected behaviour: When hovering over the data points a tooltip should be shown.
Experienced behaviour: Tooltip is not shown.

Tested on:
0.12.2 on Windows
0.11.1 on Linux (Arch)

The respective code lies in ScatterPlotItem.py (l. 1224):

           # Show information about hovered points in a tool tip
            vb = self.getViewBox()
            if vb is not None and self.opts['tip'] is not None:
                cutoff = 3
                tip = [self.opts['tip'](x=pt.pos().x(), y=pt.pos().y(), data=pt.data())
                       for pt in points[:cutoff]]
                if len(points) > cutoff:
                    tip.append('({} others...)'.format(len(points) - cutoff))
                print('\n\n'.join(tip)) # ADDED to TEST
                vb.setToolTip('\n\n'.join(tip))

            self.sigHovered.emit(self, points, ev)

The added line shows that the tip is created, but it is never shown.